[clang] [Clang][Modules] Ensure global diagnostic overrides are respected in … (PR #180684)

[Nico Weber via cfe-commits]

nico wrote:

Should the test also include a warning that has `ShowInSystemHeader` set? Wdeprecated hasn't, as far as I can tell. (See e.g. 955bb84090c13ad03017cea9ded2d19a819e90c9)

https://github.com/llvm/llvm-project/pull/180684