[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:

Ah. That's a pity but no problem. I'll create the PR now. Thanks! 

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