Our generated ObjC code should use API_AVAILABLE annotations where appropriate. For example, block trampolines should be annotated with API_AVAILABLE corresponding to the types in the block's signature.

The thing that makes this non-trivial is that a block can have arbitrarily many types in its signature, so we'll need to union the availability annotations from all of them. Best way to do this would be to add a method on Type that calculates the ApiAvailability of the type.

Related to #2904