[ObjC] Add compatibility mode for type checking of qualified id block parameters.

Commit 73152a2e fixed type checking for
blocks with qualified id parameters. But there are existing APIs in
Apple SDKs relying on the old type checking behavior. Specifically,
these are APIs using NSItemProviderCompletionHandler in
Foundation/NSItemProvider.h. To keep existing code working and to allow
developers to use affected APIs introduce a compatibility mode that
enables the previous and the fixed type checking. This mode is enabled
only on Darwin platforms.

Reviewed By: jyknight, ahatanak

Differential Revision: https://reviews.llvm.org/D79511