[clangd] Make use of diagnostic tags for some clang diags

It is not great to list diag ids by hand, but I don't see any other
solution unless diagnostics are annotated with these explicitly, which is a
bigger change in clang and I am not sure if would be worth it.

Diagnostics handled by this patch is by no means exhaustive, there might be
other checks that don't mention "unused"/"deprecated" in their names. But it
feels like this should be enough to catch common diagnostics and can be extended
over time.

Differential Revision: https://reviews.llvm.org/D107040