[libclang] Workaround potential race condition with code completion AllocatedResults being freed after a CXTranslationUnit.

The Container USR's CXString had its underlying data owned by the CXTranslationUnit's string pool.  This
would result in trying to access freed memory.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137887 91177308-0d34-0410-b5e6-96231b3b80d8