[clang] [clang][Driver] Add a custom error option in multilib.yaml. (PR #105684)

[Petr Hosek via cfe-commits]

petrhosek wrote:

I think `Error` and `Warning` would be more consistent actually, especially if `-Werror` also applied to `Warning`; ideally we would also have a custom warning type so you could do e.g. `-Wno-multilib-warning` to suppress these warnings.

https://github.com/llvm/llvm-project/pull/105684