[SourceKit] Add test case for crash triggered in swift::Mangle::Mangler::mangleAssociatedTypeName(swift::DependentMemberType*, bool)

Stack trace:

```
found code completion token A at offset 167
4  swift-ide-test  0x0000000000b6105e swift::Mangle::Mangler::mangleAssociatedTypeName(swift::DependentMemberType*, bool) + 174
5  swift-ide-test  0x0000000000b5ecb2 swift::Mangle::Mangler::mangleType(swift::Type, swift::ResilienceExpansion, unsigned int) + 3346
6  swift-ide-test  0x0000000000b6027e swift::Mangle::Mangler::mangleDeclType(swift::ValueDecl const*, swift::ResilienceExpansion, unsigned int) + 238
7  swift-ide-test  0x0000000000b5cf10 swift::Mangle::Mangler::mangleNominalType(swift::NominalTypeDecl const*, swift::ResilienceExpansion, swift::Mangle::Mangler::BindGenerics, swift::CanGenericSignature, swift::GenericParamList const*) + 256
8  swift-ide-test  0x0000000000b9b942 swift::ide::printDeclUSR(swift::ValueDecl const*, llvm::raw_ostream&) + 722
10 swift-ide-test  0x0000000000773a18 copyAssociatedUSRs(llvm::BumpPtrAllocatorImpl<llvm::MallocAllocator, 4096ul, 4096ul>&, swift::Decl const*) + 104
11 swift-ide-test  0x0000000000774198 swift::ide::CodeCompletionResultBuilder::takeResult() + 1624
14 swift-ide-test  0x0000000000b577db swift::lookupVisibleDecls(swift::VisibleDeclConsumer&, swift::DeclContext const*, swift::LazyResolver*, bool, swift::SourceLoc) + 555
18 swift-ide-test  0x0000000000ae2154 swift::Decl::walk(swift::ASTWalker&) + 20
19 swift-ide-test  0x0000000000b6be0e swift::SourceFile::walk(swift::ASTWalker&) + 174
20 swift-ide-test  0x0000000000b6b03f swift::ModuleDecl::walk(swift::ASTWalker&) + 79
21 swift-ide-test  0x0000000000b451a2 swift::DeclContext::walkContext(swift::ASTWalker&) + 146
22 swift-ide-test  0x000000000085cd1a swift::performDelayedParsing(swift::DeclContext*, swift::PersistentParserState&, swift::CodeCompletionCallbacksFactory*) + 138
23 swift-ide-test  0x000000000076bb24 swift::CompilerInstance::performSema() + 3316
24 swift-ide-test  0x00000000007152b7 main + 33239
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 b at <INPUT-FILE>:3:6
```

Author: practicalswift

validation-test/IDE/crashers/051-swift-mangle-mangler-mangleassociatedtypename.swift

@@ -0,0 +1,3 @@
+// RUN: not --crash %target-swift-ide-test -code-completion -code-completion-token=A -source-filename=%s
+// REQUIRES: asserts
+var b{A{func f<T{let H:T.c{struct S<where#^A^#