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

[Petr Hosek via cfe-commits]

petrhosek wrote:

> Would you accept `Type: Exclusive` instead of `Exclusive: True`? It seems more plausible to me that there might be three kinds of group that _can't_ go together than three group-type flags that you can have in any combination.

I like this as it scales better to other types of groups we might have in the future.

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