[clang-rename] introduce better symbol finding

This patch introduces:

* TypeLoc visiting, which helps a lot in renaming types
* NestedNameSpecifierLoc visiting (through getting them via ASTMatcher at the moment, though, because RecursiveASTVisitor<T>::VisitNestedNameSpecifierLoc isn't implemented), which helps to treat nested names correctly
* better code formatting and refactoring
* bunch of tests

Reviewers: alexfh

Differential revision: https://reviews.llvm.org/D22465

llvm-svn: 276414