[clang] [Clang][Driver] Fix inverted diagnostic condition (PR #225640)

[Michael Kruse via cfe-commits]

Meinersbur wrote:

That PR only added a test for warn_drv_input_file_unused_by_cpp, but not warn_drv_preprocessed_input_file_unused (which existed before 403f76ee02a3151a9eb2bf7b379a308da2a919ea bit changed its message).

I can change the string of warn_drv_preprocessed_input_file_unused without any test failing.

https://github.com/llvm/llvm-project/pull/225640