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

[Simon Tatham via cfe-commits]

statham-arm wrote:

For the record, that also looks spurious: again it's completely unrelated to this patch, and also, [build 4752](https://lab.llvm.org/buildbot/#/builders/123/builds/4752) of the same buildbot failed in the same way, two commits earlier. It looks more likely that that test is just flaky.


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