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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

@dwblaikie The flag is for our own use within LLVM test suite. With the previous change, we were unconditionally weakening standards conformance and runtime checking, because there are a bunch of bugs which are believed to be mostly harmless. However, if we do that without some sort of switch, we can't keep testing and fixing things on our end, so that we can some day revert to the correct behavior.

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