[clang-rename] handle overridden functions correctly

1. Renaming overridden functions only works for two levels of "overriding
   hierarchy". clang-rename should recursively add overridden methods.
2. Make use of forEachOverridden AST Matcher.
3. Fix two tests.

Reviewers: alexfh

Differential Revision: https://reviews.llvm.org/D23009

llvm-svn: 277356