[clangd] Return TextEdits from ClangdServer::applyTweak

Summary:
Instead of `tooling::Replacements`. So that embedders do not need to store
the contents of the file.

This also aligns better with `ClangdServer::rename`.

Reviewers: kadircet, hokein

Reviewed By: hokein

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63140

llvm-svn: 363150