Optional swift_newtype types are @objc if the raw type would be @objc.

[jrose-apple]

• Explanation: We import certain typedefs as if they were structs wrapping the underlying type; clearly we want to treat these as Objective-C-compatible. However, whether or not the optional version of these types is Objective-C-compatible should be dependent on whether the underlying type can be optional; we were instead rejecting it out of hand. In practice, this was causing issues where the compiler rejected overrides of imported members as being non-ObjC-compatible, even though the type was exactly the same as what the Clang importer was using.
• Scope: Affects any (1) imported struct or enum types, that (2) is used in an optional type in (3) a member that may be exposed to Objective-C.
• Issue: SR-2344
• Reviewed by: @DougGregor
• Risk: Medium-low. It's mostly existing code paths, but it does expand the inputs to that code. There's no real workaround, though.
• Testing: Added new compiler regression tests.

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