[Tooling] Add more scope specifiers until spelling is not ambiguous.

Summary:
Previously, when the renamed spelling is ambiguous, we simply use the
full-qualfied name (with leading "::"). This patch makes it try adding
additional specifiers one at a time until name is no longer ambiguous,
which allows us to find better disambuguated spelling.

Reviewers: kadircet, gribozavr

Subscribers: jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59487

llvm-svn: 356446