[clang] [clang][driver][clang-cl] Fix unused argument warning for `/std:c++20` for precompiled module inputs to `clang-cl` (PR #99300)

[Sharadh Rajaraman via cfe-commits]

sharadhr wrote:

Not quite sure how to handle the test for this; I'm struggling to find a `.pcm` file that can be used.

https://github.com/llvm/llvm-project/pull/99300