[swiftc (33 vs. 5565)] Add crasher in swift::GenericSignatureBuilder::FloatingRequirementSource::getSource(...)

Add test case for crash triggered in `swift::GenericSignatureBuilder::FloatingRequirementSource::getSource(...)`.

Current number of unresolved compiler crashers: 33 (5565 resolved)

Stack trace:

```
0 0x0000000003a7b018 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x3a7b018)
1 0x0000000003a7b756 SignalHandler(int) (/path/to/swift/bin/swift+0x3a7b756)
2 0x00007fbac22ee390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
3 0x0000000001572959 formProtocolRelativeType(swift::ProtocolDecl*, swift::GenericSignatureBuilder::PotentialArchetype*, swift::GenericSignatureBuilder::PotentialArchetype*) (/path/to/swift/bin/swift+0x1572959)
4 0x0000000001572970 formProtocolRelativeType(swift::ProtocolDecl*, swift::GenericSignatureBuilder::PotentialArchetype*, swift::GenericSignatureBuilder::PotentialArchetype*) (/path/to/swift/bin/swift+0x1572970)
5 0x00000000015727a6 swift::GenericSignatureBuilder::FloatingRequirementSource::getSource(swift::GenericSignatureBuilder::PotentialArchetype*) const (/path/to/swift/bin/swift+0x15727a6)
6 0x000000000157fb94 swift::GenericSignatureBuilder::addTypeRequirement(llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, swift::GenericSignatureBuilder::FloatingRequirementSource, swift::GenericSignatureBuilder::UnresolvedHandlingKind) (/path/to/swift/bin/swift+0x157fb94)
7 0x000000000157d62b swift::GenericSignatureBuilder::addRequirement(swift::Requirement const&, swift::GenericSignatureBuilder::FloatingRequirementSource, swift::ModuleDecl*, swift::SubstitutionMap const*) (/path/to/swift/bin/swift+0x157d62b)
8 0x00000000015896dd swift::GenericSignatureBuilder::InferRequirementsWalker::walkToTypePost(swift::Type) (/path/to/swift/bin/swift+0x15896dd)
9 0x00000000015e115c swift::TypeVisitor<(anonymous namespace)::Traversal, bool>::visit(swift::Type) (/path/to/swift/bin/swift+0x15e115c)
10 0x00000000015e117f swift::TypeVisitor<(anonymous namespace)::Traversal, bool>::visit(swift::Type) (/path/to/swift/bin/swift+0x15e117f)
11 0x00000000015e117f swift::TypeVisitor<(anonymous namespace)::Traversal, bool>::visit(swift::Type) (/path/to/swift/bin/swift+0x15e117f)
12 0x00000000015e0134 swift::Type::walk(swift::TypeWalker&) const (/path/to/swift/bin/swift+0x15e0134)
13 0x0000000001590bae swift::GenericSignatureBuilder::ConstraintResult llvm::function_ref<swift::GenericSignatureBuilder::ConstraintResult (swift::Type, swift::TypeRepr const*)>::callback_fn<swift::GenericSignatureBuilder::addInheritedRequirements(swift::TypeDecl*, llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, swift::GenericSignatureBuilder::RequirementSource const*, swift::ModuleDecl*)::$_26>(long, swift::Type, swift::TypeRepr const*) (/path/to/swift/bin/swift+0x1590bae)
14 0x0000000001589522 std::_Function_handler<void (swift::Type, swift::TypeRepr const*), visitInherited(llvm::ArrayRef<swift::TypeLoc>, llvm::function_ref<swift::GenericSignatureBuilder::ConstraintResult (swift::Type, swift::TypeRepr const*)>)::$_59>::_M_invoke(std::_Any_data const&, swift::Type&&, swift::TypeRepr const*&&) (/path/to/swift/bin/swift+0x1589522)
15 0x000000000157b6a1 swift::GenericSignatureBuilder::addInheritedRequirements(swift::TypeDecl*, llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, swift::GenericSignatureBuilder::RequirementSource const*, swift::ModuleDecl*) (/path/to/swift/bin/swift+0x157b6a1)
16 0x000000000157bc74 swift::GenericSignatureBuilder::addConformanceRequirement(swift::GenericSignatureBuilder::PotentialArchetype*, swift::ProtocolDecl*, swift::GenericSignatureBuilder::RequirementSource const*) (/path/to/swift/bin/swift+0x157bc74)
17 0x000000000157fc55 swift::GenericSignatureBuilder::addTypeRequirement(llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, swift::GenericSignatureBuilder::FloatingRequirementSource, swift::GenericSignatureBuilder::UnresolvedHandlingKind) (/path/to/swift/bin/swift+0x157fc55)
18 0x0000000001590d41 swift::GenericSignatureBuilder::ConstraintResult llvm::function_ref<swift::GenericSignatureBuilder::ConstraintResult (swift::Type, swift::TypeRepr const*)>::callback_fn<swift::GenericSignatureBuilder::addInheritedRequirements(swift::TypeDecl*, llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, swift::GenericSignatureBuilder::RequirementSource const*, swift::ModuleDecl*)::$_26>(long, swift::Type, swift::TypeRepr const*) (/path/to/swift/bin/swift+0x1590d41)
19 0x0000000001589522 std::_Function_handler<void (swift::Type, swift::TypeRepr const*), visitInherited(llvm::ArrayRef<swift::TypeLoc>, llvm::function_ref<swift::GenericSignatureBuilder::ConstraintResult (swift::Type, swift::TypeRepr const*)>)::$_59>::_M_invoke(std::_Any_data const&, swift::Type&&, swift::TypeRepr const*&&) (/path/to/swift/bin/swift+0x1589522)
20 0x000000000157b6a1 swift::GenericSignatureBuilder::addInheritedRequirements(swift::TypeDecl*, llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, swift::GenericSignatureBuilder::RequirementSource const*, swift::ModuleDecl*) (/path/to/swift/bin/swift+0x157b6a1)
21 0x000000000157b4be swift::GenericSignatureBuilder::addGenericParameterRequirements(swift::GenericTypeParamDecl*) (/path/to/swift/bin/swift+0x157b4be)
22 0x0000000001385a22 swift::TypeChecker::checkGenericParamList(swift::GenericSignatureBuilder*, swift::GenericParamList*, swift::GenericSignature*, swift::GenericTypeResolver*) (/path/to/swift/bin/swift+0x1385a22)
23 0x0000000001389004 swift::TypeChecker::checkGenericEnvironment(swift::GenericParamList*, swift::DeclContext*, swift::GenericSignature*, bool, llvm::function_ref<void (swift::GenericSignatureBuilder&)>) (/path/to/swift/bin/swift+0x1389004)
24 0x0000000001389429 swift::TypeChecker::validateGenericTypeSignature(swift::GenericTypeDecl*) (/path/to/swift/bin/swift+0x1389429)
25 0x000000000135a200 swift::TypeChecker::validateDecl(swift::ValueDecl*) (/path/to/swift/bin/swift+0x135a200)
26 0x0000000001369ee3 (anonymous namespace)::DeclChecker::visitProtocolDecl(swift::ProtocolDecl*) (/path/to/swift/bin/swift+0x1369ee3)
27 0x0000000001358444 (anonymous namespace)::DeclChecker::visit(swift::Decl*) (/path/to/swift/bin/swift+0x1358444)
28 0x000000000136915b (anonymous namespace)::DeclChecker::visitClassDecl(swift::ClassDecl*) (/path/to/swift/bin/swift+0x136915b)
29 0x000000000135853e (anonymous namespace)::DeclChecker::visit(swift::Decl*) (/path/to/swift/bin/swift+0x135853e)
30 0x00000000013689fb (anonymous namespace)::DeclChecker::visitExtensionDecl(swift::ExtensionDecl*) (/path/to/swift/bin/swift+0x13689fb)
31 0x0000000001358474 (anonymous namespace)::DeclChecker::visit(swift::Decl*) (/path/to/swift/bin/swift+0x1358474)
32 0x0000000001358343 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) (/path/to/swift/bin/swift+0x1358343)
33 0x00000000013e3344 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int) (/path/to/swift/bin/swift+0x13e3344)
34 0x0000000000fa16cd swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0xfa16cd)
35 0x00000000004abe59 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) (/path/to/swift/bin/swift+0x4abe59)
36 0x00000000004aa3f9 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x4aa3f9)
37 0x0000000000465697 main (/path/to/swift/bin/swift+0x465697)
38 0x00007fbac07fe830 __libc_start_main /build/glibc-bfm8X4/glibc-2.23/csu/../csu/libc-start.c:325:0
39 0x0000000000462d39 _start (/path/to/swift/bin/swift+0x462d39)
```

Author: practicalswift

validation-test/compiler_crashers/28791-formprotocolrelativetype-swift-protocoldecl-swift-genericsignaturebuilder-potent.swift

@@ -0,0 +1,9 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+extension CountableRange{class a{protocol P{protocol P{}typealias e:P