[6.0][IRGen+Runtime] Fix tag bit mask handling for objc, unknown objects a…

…nd blocks

Explanation: On platforms that don't have reserved bits in objc (including unknown) pointers, we use the spare bits for Swift enums, so they have to be masked out. Blocks don't have reserved bits on any platform.
Scope: Compact value witnesses runtime.
Issues: rdar://138487964, rdar://139830282
Original PRs: https://github.com/swiftlang/swift/pull/77431
Risk: Low. Only touches cases that are already broken today and only on the affected platforms.
Testing: Added regression tests.
Reviewers: @mikeash

swiftlang / swift