[swiftc] Add test case for crash triggered in swift::TypeChecker::resolveTypeWitness(swift::NormalProtocolConformance const*, swift::AssociatedTypeDecl*)

Stack trace:

```
5  swift           0x0000000000e41101 swift::TypeChecker::resolveTypeWitness(swift::NormalProtocolConformance const*, swift::AssociatedTypeDecl*) + 225
6  swift           0x00000000010341a6 swift::NormalProtocolConformance::getTypeWitnessSubstAndDecl(swift::AssociatedTypeDecl*, swift::LazyResolver*) const + 150
7  swift           0x00000000010340e8 swift::ProtocolConformance::getTypeWitnessSubstAndDecl(swift::AssociatedTypeDecl*, swift::LazyResolver*) const + 40
8  swift           0x00000000010348e6 swift::ProtocolConformance::getTypeWitness(swift::AssociatedTypeDecl*, swift::LazyResolver*) const + 6
11 swift           0x0000000000f1f058 swift::ArchetypeBuilder::addSuperclassRequirement(swift::ArchetypeBuilder::PotentialArchetype*, swift::Type, swift::RequirementSource) + 248
14 swift           0x0000000000f20d4f swift::ArchetypeBuilder::visitInherited(llvm::ArrayRef<swift::TypeLoc>, llvm::function_ref<bool (swift::Type, swift::SourceLoc)>) + 175
15 swift           0x0000000000f1eb4a swift::ArchetypeBuilder::addAbstractTypeParamRequirements(swift::AbstractTypeParamDecl*, swift::ArchetypeBuilder::PotentialArchetype*, swift::RequirementSource::Kind, llvm::SmallPtrSetImpl<swift::ProtocolDecl*>&) + 426
16 swift           0x0000000000f1eed7 swift::ArchetypeBuilder::addConformanceRequirement(swift::ArchetypeBuilder::PotentialArchetype*, swift::ProtocolDecl*, swift::RequirementSource, llvm::SmallPtrSetImpl<swift::ProtocolDecl*>&) + 471
19 swift           0x0000000000f20d4f swift::ArchetypeBuilder::visitInherited(llvm::ArrayRef<swift::TypeLoc>, llvm::function_ref<bool (swift::Type, swift::SourceLoc)>) + 175
20 swift           0x0000000000f1eb4a swift::ArchetypeBuilder::addAbstractTypeParamRequirements(swift::AbstractTypeParamDecl*, swift::ArchetypeBuilder::PotentialArchetype*, swift::RequirementSource::Kind, llvm::SmallPtrSetImpl<swift::ProtocolDecl*>&) + 426
21 swift           0x0000000000f1e97c swift::ArchetypeBuilder::addGenericParameterRequirements(swift::GenericTypeParamDecl*) + 172
22 swift           0x0000000000e34427 swift::TypeChecker::checkGenericParamList(swift::ArchetypeBuilder*, swift::GenericParamList*, swift::GenericSignature*, bool, swift::GenericTypeResolver*) + 375
23 swift           0x0000000000e35b37 swift::TypeChecker::validateGenericSignature(swift::GenericParamList*, swift::DeclContext*, swift::GenericSignature*, std::function<bool (swift::ArchetypeBuilder&)>, bool&) + 135
24 swift           0x0000000000e35ee4 swift::TypeChecker::validateGenericTypeSignature(swift::NominalTypeDecl*) + 116
25 swift           0x0000000000df9bd4 swift::TypeChecker::validateDecl(swift::ValueDecl*, bool) + 1300
28 swift           0x0000000000dff206 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) + 150
29 swift           0x0000000000e22092 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int) + 1026
30 swift           0x0000000000c97b9f swift::CompilerInstance::performSema() + 3167
32 swift           0x000000000077705b frontend_main(llvm::ArrayRef<char const*>, char const*, void*) + 2523
33 swift           0x0000000000771b25 main + 2773
Stack dump:
0.	Program arguments: /usr/local/bin/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28257-swift-typechecker-resolvetypewitness.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28257-swift-typechecker-resolvetypewitness-eec025.o
1.	While type-checking 'A' at validation-test/compiler_crashers/28257-swift-typechecker-resolvetypewitness.swift:7:1
<unknown>:0: error: unable to execute command: Segmentation fault
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```

Author: practicalswift

validation-test/compiler_crashers/28257-swift-typechecker-resolvetypewitness.swift

@@ -0,0 +1,14 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+
+// Distributed under the terms of the MIT license
+// Test case submitted to project by https://github.com/practicalswift (practicalswift)
+// Test case found by fuzzing
+
+protocol A{
+class A:a{
+}
+typealias e:a
+protocol a{
+typealias r
+}
+typealias e:A