[clangd] Ignore diags from builtin files

Summary:
This fixes a case where we show diagnostics on arbitrary lines, in an
internal codebase.

Open for ideas on unittesting this.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64863

llvm-svn: 367303