[clang-tidy] Implement FixitHints for identifier references in IdentifierNamingCheck

This diff requires http://reviews.llvm.org/D13079 to be applied first. I wasn't sure about how to make patch series in Phabricator, and I wanted to keep the two separate for clarity.

It looks like that most cases can be supported with this patch. I'm not totally sure about the actual coverage though. I think that the matchers are very generic, but I'm still not totally fluent with the AST.

Patch by Beren Minor!

Differential revision: http://reviews.llvm.org/D13081

llvm-svn: 248996