[clang] [Sema] Suggest missing format attributes (PR #166738)

[via cfe-commits]

apple-fcloutier wrote:

> Do you think it would be better to disable checking for missing attributes in the existing `format-strings` tests and instead create a separate test that uses `-Wformat-nonliteral` both with and without `-Wmissing-format-attribute` to compare them?

I don't think that's necessary, the format test files are already kind of a hodgepodge of all things format and this area sees changes maybe once or twice a year.

https://github.com/llvm/llvm-project/pull/166738