[libclang] When caching code-completion results, pass the CachedCompletionAllocator

to the CodeCompletionTUInfo that is going to be used to get the results.

Previously we would use ASTUnit's CodeCompletionTUInfo which has its own allocator
that will go away when we reparse. That could result in a use-after-free bug when
getting the parent context name from a CodeCompletionString.

Addresses rdar://12568377.

llvm-svn: 168133