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

[Simon Tatham via cfe-commits]

statham-arm wrote:

(This final force-push is the squashed version of the previous stack, rebased to the current head of `main`, so that the builder can run a last test. Thanks both for the approvals; I'll merge it once the tests have finished.)

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