[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:

Just to clarify—you'd like me to open another PR to `release/19.X`? I can do that.

In fact I'm a little more ambitious because this and #98761 are honestly very simple PRs—I'd like them to be backported as far as Clang 16 (which is when modules support first appeared) if possible. 

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