swift234

[swiftc] Add test case for crash triggered in swift::ValueDecl::setType(swift::Type)

Stack trace:

```
swift: /path/to/swift/lib/AST/Decl.cpp:1655: void swift::ValueDecl::setType(swift::Type): Assertion `!hasType() && "changing type of declaration"' failed.
8  swift           0x000000000108473c swift::ValueDecl::setType(swift::Type) + 92
9  swift           0x0000000000e788cc swift::configureConstructorType(swift::ConstructorDecl*, swift::Type, swift::Type, bool) + 428
14 swift           0x0000000000e7bf36 swift::TypeChecker::typeCheckDecl(swift::Decl*, bool) + 150
15 swift           0x0000000000e9d962 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int) + 994
16 swift           0x0000000000c37c0f swift::CompilerInstance::performSema() + 3295
18 swift           0x00000000007c5e46 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2838
19 swift           0x0000000000791ed8 main + 2840
Stack dump:
0.	Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28292-swift-valuedecl-settype.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28292-swift-valuedecl-settype-ee60c6.o
1.	While type-checking 'a' at validation-test/compiler_crashers/28292-swift-valuedecl-settype.swift:10:1
<unknown>:0: error: unable to execute command: Aborted
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```