[clang] [C++20] [Modules] Introduce -fskip-odr-check-in-gmf (PR #79959)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> I'd still idly vote against adding this flag/support - but if other modules contributors feel it's the right thing to do, I won't stand in the way.

Yeah, as @mizvekov said, this is intended to be a transparent change to users. (unless the users are testing volunteers, which is highly welcomed)

https://github.com/llvm/llvm-project/pull/79959