When back-deploying, create global-actor-qualified function types via a
separate entrypoint
(`swift_getFunctionTypeMetadataGlobalActorBackDeploy`) in the
compatibility library, which checks whether it is running with a
new-enough runtime to use `swift_getFunctionTypeMetadataGlobalActor`.
Failing that, it calls into a separate copy of the implementation that
exists only in the back-deployed concurrency library.
Fixes rdar://79153988.
