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

[Petr Hosek via cfe-commits]

petrhosek wrote:

This is just a nit, but is there a reason for calling this `FatalError` rather than just `Error`? To me "fatal" usually means something went seriously wrong, but in this case it's just the fact the combination of flags is not supported and in the driver we usually just refer to these cases as errors.

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