[clang] [clang] Allow MCTargetOptions to be parseable by -mllvm. (PR #66347)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay requested changes to this pull request.

Agree that the motivation should be communicated. Driver options have stronger stability guarantee. If there are sufficient motivation and useful MCTargetOptions that we don't feel comfortable exposing driver options, I think this is probably fine.

We probably don't want to test every option. Just 2 or 3 suffices.

https://github.com/llvm/llvm-project/pull/66347