[Clangd] Fixed clangd diagnostics priority

Summary:
- Fixed diagnostics where zero width inserted ranges were being used instead of the whole token
- Added unit tests

Patch by @SureYeaah !

Reviewers: sammccall, kadircet

Reviewed By: kadircet

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

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D63222

llvm-svn: 363253