[clang] [clang][Modules] Respect -fno-cxx-modules as a driver flag (PR #150349)

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

> If we want, I think we can introduce a new language options for C++20 modules only. If it enabled, the C++20 modules are enabled. If it is disabled, the C++20 modules will be disabled.

Thanks for offering such this suggestion, but I'm not sure if that's necessary _yet_. If the lack of distinction actively causes adoption issues for either clang modules or c++20 modules, then I'd say yes, but I'm not aware of this. In an ideal world, I'd hope these two definitions of modules can be enabled & intermixed without issues,[ especially if header units end up being represented with modulemaps (or something like it)](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3696r0.html).

FWIW, I actually noticed the issue this PR fixes when trying to clean up some of the Darwin-toolchain compiler divergence around this area.


https://github.com/llvm/llvm-project/pull/150349