This commit changes fixit messages from a question/suggestion to an
imperative message for protocol conformances and switch-case. Addresses
https://github.com/apple/swift/issues/67510.
While a Swift protocol can have its async requirements satisfied
by a sync witness, this is not the case for ObjC protocols because
it is not just a calling convention difference.
Because every async ObjC function requirement in the protocol also
has a sibling that is sync, a sync witness might be trying
to conform to the sync version that takes a completion handler.
Without this change, we were seeing an issue where we would
consider both the async and sync versions of an ObjC requirement,
and accidentially choose the async version when the witness was
sync, and then raise an error about the typechecker's mistake.
Instead of raising an error, this change removes the ability for
the typechecker to even consider the errornous conformance.
resolves rdar://73326234
