[SourceKit] Add test case for crash triggered in swift::Mangle::Mangler::mangleIdentifier(swift::Identifier, swift::Mangle::OperatorFixity)

Stack trace:

```
found code completion token A at offset 122
swift-ide-test: /path/to/llvm/include/llvm/ADT/StringRef.h:73: llvm::StringRef::StringRef(const char *): Assertion `Str && "StringRef cannot be built from a NULL argument"' failed.
8  swift-ide-test  0x0000000000b60404 swift::Mangle::Mangler::mangleIdentifier(swift::Identifier, swift::Mangle::OperatorFixity) + 164
9  swift-ide-test  0x0000000000b61de2 swift::Mangle::Mangler::mangleDeclName(swift::ValueDecl const*) + 850
10 swift-ide-test  0x0000000000b6174c swift::Mangle::Mangler::mangleEntity(swift::ValueDecl const*, swift::ResilienceExpansion, unsigned int) + 428
11 swift-ide-test  0x0000000000b61741 swift::Mangle::Mangler::mangleEntity(swift::ValueDecl const*, swift::ResilienceExpansion, unsigned int) + 417
12 swift-ide-test  0x0000000000b9fe7f swift::ide::printDeclUSR(swift::ValueDecl const*, llvm::raw_ostream&) + 815
14 swift-ide-test  0x000000000077bd88 copyAssociatedUSRs(llvm::BumpPtrAllocatorImpl<llvm::MallocAllocator, 4096ul, 4096ul>&, swift::Decl const*) + 104
15 swift-ide-test  0x000000000077c508 swift::ide::CodeCompletionResultBuilder::takeResult() + 1624
19 swift-ide-test  0x0000000000b5bd5b swift::lookupVisibleDecls(swift::VisibleDeclConsumer&, swift::DeclContext const*, swift::LazyResolver*, bool, swift::SourceLoc) + 555
24 swift-ide-test  0x0000000000ae6734 swift::Decl::walk(swift::ASTWalker&) + 20
25 swift-ide-test  0x0000000000b6fbce swift::SourceFile::walk(swift::ASTWalker&) + 174
26 swift-ide-test  0x0000000000b6efaf swift::ModuleDecl::walk(swift::ASTWalker&) + 79
27 swift-ide-test  0x0000000000b49722 swift::DeclContext::walkContext(swift::ASTWalker&) + 146
28 swift-ide-test  0x000000000086599d swift::performDelayedParsing(swift::DeclContext*, swift::PersistentParserState&, swift::CodeCompletionCallbacksFactory*) + 61
29 swift-ide-test  0x0000000000774304 swift::CompilerInstance::performSema() + 3316
30 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 walking into decl getter for d at <INPUT-FILE>:2:6
```

Author: practicalswift

validation-test/IDE/crashers/016-swift-mangle-mangler-mangleidentifier.swift

@@ -0,0 +1,2 @@
+// RUN: not --crash %target-swift-ide-test -code-completion -code-completion-token=A -source-filename=%s
+var d{let:{func a#^A^#