[clangd] Revert back to previous heuristic for diagnostic range extraction.

Summary: Also add a few new test cases and a special case into handling of empty fixit ranges that  collides with location of a diag.

Reviewers: sammccall, ilya-biryukov

Reviewed By: sammccall

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D52889

llvm-svn: 344025