[code-complete] Introduce CodeCompletionTUInfo which will be used for caching

code-completion related strings specific to a translation unit (ASTContext and related data)

CodeCompletionAllocator does such limited caching, by caching the name assigned
to a DeclContext*, but that is not the appropriate place since that object has
a lifetime that can extend beyond that of an ASTContext.

Introduce CodeCompletionTUInfo which will be always tied to a translation unit
to do this kind of caching and move the caching of CodeCompletionAllocator into this
object, and propagate it to all the places where it will be needed.

The plan is to extend the caching where appropriate, using CodeCompletionTUInfo,
to avoid re-calculating code-completion strings.

Part of rdar://10796159.

llvm-svn: 154408