[clangd] Add rename support.

Summary:
Make clangd handle "textDocument/rename" request. The rename
functionality comes from the "local-rename" sub-tool of clang-refactor.

Currently clangd only supports local rename (only symbol occurrences in
the main file will be renamed).

Reviewers: sammccall, ilya-biryukov

Reviewed By: sammccall

Subscribers: cfe-commits, ioeric, arphaman, mgorny

Differential Revision: https://reviews.llvm.org/D39676

llvm-svn: 317780