[clangd] Use raw rename functions to implement the rename.

Summary:
The API provided by refactoring lib doesn't provide enough flexibility
to get clangd's rename to behave as we expect. Instead, we replace it
with the low-level rename functions, which give us more control.

Bonus:
- performance, previously we visit the TU to find all occurrences,
  now we just visit top-level decls from main file;
- fix a bug where we wrongly filter out the main file replacement due to the
  different relative/absolute file path;

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, jfb, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65936

llvm-svn: 368429