[swiftc (31 vs. 5563)] Add crasher in swift::TypeBase::getContextSubstitutions

Add test case for crash triggered in `swift::TypeBase::getContextSubstitutions`.

Current number of unresolved compiler crashers: 31 (5563 resolved)

Stack trace:

```
0 0x0000000003a765e8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x3a765e8)
1 0x0000000003a76d26 SignalHandler(int) (/path/to/swift/bin/swift+0x3a76d26)
2 0x00007f8cf176b390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
3 0x00000000015cef7b swift::TypeBase::getContextSubstitutions(swift::DeclContext const*, swift::GenericEnvironment*) (/path/to/swift/bin/swift+0x15cef7b)
4 0x00000000015ca9c0 swift::TypeBase::getContextSubstitutionMap(swift::ModuleDecl*, swift::DeclContext const*, swift::GenericEnvironment*) (/path/to/swift/bin/swift+0x15ca9c0)
5 0x0000000001445fbf swift::createDesignatedInitOverride(swift::TypeChecker&, swift::ClassDecl*, swift::ConstructorDecl*, swift::DesignatedInitKind) (/path/to/swift/bin/swift+0x1445fbf)
6 0x000000000135cd06 swift::TypeChecker::addImplicitConstructors(swift::NominalTypeDecl*) (/path/to/swift/bin/swift+0x135cd06)
7 0x0000000001365f65 (anonymous namespace)::DeclChecker::visitClassDecl(swift::ClassDecl*) (/path/to/swift/bin/swift+0x1365f65)
8 0x000000000135538e (anonymous namespace)::DeclChecker::visit(swift::Decl*) (/path/to/swift/bin/swift+0x135538e)
9 0x0000000001365e3b (anonymous namespace)::DeclChecker::visitStructDecl(swift::StructDecl*) (/path/to/swift/bin/swift+0x1365e3b)
10 0x0000000001355274 (anonymous namespace)::DeclChecker::visit(swift::Decl*) (/path/to/swift/bin/swift+0x1355274)
11 0x000000000136584b (anonymous namespace)::DeclChecker::visitExtensionDecl(swift::ExtensionDecl*) (/path/to/swift/bin/swift+0x136584b)
12 0x00000000013552c4 (anonymous namespace)::DeclChecker::visit(swift::Decl*) (/path/to/swift/bin/swift+0x13552c4)
13 0x0000000001355193 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) (/path/to/swift/bin/swift+0x1355193)
14 0x00000000013dff8a swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int) (/path/to/swift/bin/swift+0x13dff8a)
15 0x0000000000f9de7d swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0xf9de7d)
16 0x00000000004abe79 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) (/path/to/swift/bin/swift+0x4abe79)
17 0x00000000004aa419 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x4aa419)
18 0x0000000000465697 main (/path/to/swift/bin/swift+0x465697)
19 0x00007f8cefc7b830 __libc_start_main /build/glibc-bfm8X4/glibc-2.23/csu/../csu/libc-start.c:325:0
20 0x0000000000462d39 _start (/path/to/swift/bin/swift+0x462d39)
```

Author: practicalswift

validation-test/compiler_crashers/28786-swift-typebase-getcontextsubstitutions-swift-declcontext-const-swift-genericenvi.swift

@@ -0,0 +1,11 @@
+// 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
+protocol P}typealias a:P}extension P{class a
+struct B{{{}}class a:A.a
+protocol A:a