[clang] [Driver] Add ExclusiveGroup feature to multilib.yaml. (PR #69447)

[Simon Tatham via cfe-commits]

statham-arm wrote:

OK, here's a version with the syntax that way. I've added another test to demonstrate the new error checks.

The implementation of exclusion is still done by having an `ExclusiveGroup` field in the actual `Multilib` class. Implementing mutually-dependent groups or nested groups is enough extra effort that I'd rather leave it until we actually need it! But now the user-facing syntax in `multilib.yaml` is futureproof against wanting to add those features later, so _only_ the implementation should need to change.

https://github.com/llvm/llvm-project/pull/69447