swift234

[swiftc] Add test case for crash triggered in swift::GenericSignature::getSubstitutionMap(llvm::ArrayRef<swift::Substitution>) const

Stack trace:

```
swift: /path/to/llvm/include/llvm/ADT/ArrayRef.h:139: const T &llvm::ArrayRef<swift::Substitution>::front() const [T = swift::Substitution]: Assertion `!empty()' failed.
8  swift           0x00000000010cc8a6 swift::GenericSignature::getSubstitutionMap(llvm::ArrayRef<swift::Substitution>) const + 566
9  swift           0x0000000000f1d41a swift::createDesignatedInitOverride(swift::TypeChecker&, swift::ClassDecl*, swift::ConstructorDecl*, swift::DesignatedInitKind) + 266
10 swift           0x0000000000ea60a7 swift::TypeChecker::addImplicitConstructors(swift::NominalTypeDecl*) + 3095
13 swift           0x0000000000e9fe76 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) + 150
16 swift           0x0000000000f0465a swift::TypeChecker::typeCheckFunctionBodyUntil(swift::FuncDecl*, swift::SourceLoc) + 346
17 swift           0x0000000000f044be swift::TypeChecker::typeCheckAbstractFunctionBodyUntil(swift::AbstractFunctionDecl*, swift::SourceLoc) + 46
18 swift           0x0000000000f05083 swift::TypeChecker::typeCheckAbstractFunctionBody(swift::AbstractFunctionDecl*) + 179
20 swift           0x0000000000ec3221 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) + 1249
21 swift           0x0000000000c58a39 swift::CompilerInstance::performSema() + 3289
23 swift           0x00000000007d6d6f swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2863
24 swift           0x00000000007a2db8 main + 2872
Stack dump:
0.	Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28296-swift-genericsignature-getsubstitutionmap.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28296-swift-genericsignature-getsubstitutionmap-92b81d.o
1.	While type-checking 'e' at validation-test/compiler_crashers/28296-swift-genericsignature-getsubstitutionmap.swift:10:45
2.	While type-checking 'A' at validation-test/compiler_crashers/28296-swift-genericsignature-getsubstitutionmap.swift:10:52
<unknown>:0: error: unable to execute command: Aborted
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```