[Parser] Suppress -Wempty-translation-unit if this is a header file

This is motivated by tooling (clangd, libclang etc) - headers without
declarations are legitimate even if they're not valid TUs.

The other use -x c-header cases (PCH/modules) are nonstandard anyway and this
warning doesn't seem necessary there either.

Differential Revision: https://reviews.llvm.org/D85789