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

[Petr Hosek via cfe-commits]

petrhosek wrote:

> To be clear, are you asking _me_ to make a followup PR to change that identifier in this already-landed patch, or are you going to do it?
> 
> (Just to avoid the situation where both of us do it, or both of us think the other is going to)

If you have time to implement it, that would be great. I might be able to take a look, but it's going to be a few weeks before I have time, and if we're going to rename `FatalError` I think we better do it as soon as possible before users start using it. I'd do it as two separate changes, first to rename `FatalError` to `Error` and second to introduce `Warning`.

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