swift234

[SourceKit] Add test case for crash triggered in swift::Expr::walk(swift::ASTWalker&)

Stack trace:

```
found code completion token A at offset 122
swift-ide-test: /path/to/llvm/include/llvm/ADT/PointerUnion.h:297: T llvm::PointerUnion3<swift::Expr *, swift::Stmt *, swift::Decl *>::get() const [PT1 = swift::Expr *, PT2 = swift::Stmt *, PT3 = swift::Decl *, T = swift::Stmt *]: Assertion `is<T>() && "Invalid accessor called"' failed.
14 swift-ide-test  0x0000000000ae636e swift::Expr::walk(swift::ASTWalker&) + 46
15 swift-ide-test  0x00000000009b9af6 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::Expr*, swift::Type, bool, bool, bool) + 502
16 swift-ide-test  0x000000000091ebdb swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*) + 683
18 swift-ide-test  0x0000000000981de6 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) + 134
19 swift-ide-test  0x000000000090907d swift::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) + 1117
23 swift-ide-test  0x0000000000865a46 swift::performDelayedParsing(swift::DeclContext*, swift::PersistentParserState&, swift::CodeCompletionCallbacksFactory*) + 230
24 swift-ide-test  0x0000000000774304 swift::CompilerInstance::performSema() + 3316
25 swift-ide-test  0x000000000071cc33 main + 35011
Stack dump:
0.	Program arguments: swift-ide-test -code-completion -code-completion-token=A -source-filename=<INPUT-FILE>
1.	While type-checking expression at [<INPUT-FILE>:2:1 - line:3:14] RangeText="[{}
({[{struct A{"
```