[swiftc] Add test case for crash triggered in swift::lookupVisibleDecls(swift::VisibleDeclConsumer&, swift::DeclContext const*, swift::LazyResolver*, bool, swift::SourceLoc)

Stack trace:

```
swift: /path/to/llvm/include/llvm/Support/Casting.h:237: typename cast_retty<X, Y *>::ret_type llvm::cast(Y *) [X = swift::ClosureExpr, Y = const swift::AbstractClosureExpr]: Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
8  swift           0x00000000010d08d3 swift::lookupVisibleDecls(swift::VisibleDeclConsumer&, swift::DeclContext const*, swift::LazyResolver*, bool, swift::SourceLoc) + 1971
9  swift           0x0000000000edbc74 swift::TypeChecker::performTypoCorrection(swift::DeclContext*, swift::DeclRefKind, swift::DeclName, swift::SourceLoc, swift::OptionSet<swift::NameLookupFlags, unsigned int>, swift::LookupResult&, unsigned int) + 260
10 swift           0x0000000000e868a8 swift::TypeChecker::resolveDeclRefExpr(swift::UnresolvedDeclRefExpr*, swift::DeclContext*) + 3864
12 swift           0x000000000104df53 swift::Expr::walk(swift::ASTWalker&) + 19
13 swift           0x0000000000e870c7 swift::TypeChecker::solveForExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::FreeTypeVariableBinding, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem&, llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 119
14 swift           0x0000000000e8da82 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*) + 610
17 swift           0x0000000000f0465a swift::TypeChecker::typeCheckFunctionBodyUntil(swift::FuncDecl*, swift::SourceLoc) + 346
18 swift           0x0000000000f044be swift::TypeChecker::typeCheckAbstractFunctionBodyUntil(swift::AbstractFunctionDecl*, swift::SourceLoc) + 46
19 swift           0x0000000000f05083 swift::TypeChecker::typeCheckAbstractFunctionBody(swift::AbstractFunctionDecl*) + 179
21 swift           0x0000000000ec3221 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) + 1249
22 swift           0x0000000000c58a39 swift::CompilerInstance::performSema() + 3289
24 swift           0x00000000007d6d6f swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2863
25 swift           0x00000000007a2db8 main + 2872
Stack dump:
0.	Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28297-swift-lookupvisibledecls.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28297-swift-lookupvisibledecls-ca545b.o
1.	While type-checking getter for a at validation-test/compiler_crashers/28297-swift-lookupvisibledecls.swift:10:13
2.	While type-checking expression at [validation-test/compiler_crashers/28297-swift-lookupvisibledecls.swift:10:14 - line:10:14] RangeText="p"
<unknown>:0: error: unable to execute command: Aborted
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```

Author: practicalswift

validation-test/compiler_crashers/28297-swift-lookupvisibledecls.swift

@@ -0,0 +1,10 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not --crash %target-swift-frontend %s -parse
+// REQUIRES: asserts
+{} ?? {let a{p