swift234

[swiftc] Add test case for crash triggered in swift::ArchetypeBuilder::getGenericSignature(llvm::ArrayRef<swift::GenericTypeParamType*>)

Stack trace:

```
swift: /path/to/llvm/include/llvm/ADT/TinyPtrVector.h:182: EltTy llvm::TinyPtrVector<swift::ArchetypeBuilder::PotentialArchetype *>::front() const [EltTy = swift::ArchetypeBuilder::PotentialArchetype *]: Assertion `!empty() && "vector empty"' failed.
10 swift           0x0000000000f78281 swift::ArchetypeBuilder::getGenericSignature(llvm::ArrayRef<swift::GenericTypeParamType*>) + 1089
11 swift           0x0000000000e885f7 swift::TypeChecker::validateGenericFuncSignature(swift::AbstractFunctionDecl*) + 359
16 swift           0x0000000000e52296 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) + 150
19 swift           0x0000000000eb203a swift::TypeChecker::typeCheckClosureBody(swift::ClosureExpr*) + 218
20 swift           0x0000000000edc00c swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::Expr*, swift::Type, bool, bool, bool) + 812
21 swift           0x0000000000e401aa swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*) + 746
23 swift           0x0000000000eb2186 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) + 134
24 swift           0x0000000000e755bd swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int) + 1117
25 swift           0x0000000000cc6e2f swift::CompilerInstance::performSema() + 3087
27 swift           0x000000000078d17c frontend_main(llvm::ArrayRef<char const*>, char const*, void*) + 2492
28 swift           0x0000000000787c45 main + 2837
Stack dump:
0.	Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28277-swift-archetypebuilder-getgenericsignature.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28277-swift-archetypebuilder-getgenericsignature-82990f.o
1.	While type-checking expression at [validation-test/compiler_crashers/28277-swift-archetypebuilder-getgenericsignature.swift:10:1 - line:13:23] RangeText="{
2.	While type-checking 'A' at validation-test/compiler_crashers/28277-swift-archetypebuilder-getgenericsignature.swift:11:1
<unknown>:0: error: unable to execute command: Aborted
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```