[Driver] [C++20] [Modules] Don't emit unused-argument warning for...

[Driver] [C++20] [Modules] Don't emit unused-argument warning for '-fmodule-output'  and '-fmodule-output='

Suppres the r unused-argument warning for -fmodule-output according to
the discussion thread in
https://gcc.gnu.org/pipermail/gcc/2022-December/240239.html.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D140001