[clangd] Only emit default error/fatal diagnostices from included files.

Summary:
This would avoid adding too much noise when there is a "-Wall" in the
compile command.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79923