[clangd] Drop diags from non-written #include.

Summary: This would fix that we show weird diagnostics on random lines of the main file.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66074

llvm-svn: 368549