[clangd] Handle clang-tidy suppression comments for diagnostics inside macro expansions

Summary:
Not handling this was a side-effect of being overly cautious when trying
to avoid reading files for which clangd doesn't have the source mapped.

Fixes https://github.com/clangd/clangd/issues/266

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75286