introduce UnderscoreOwned feature

[kavon]

This experimental feature allows you to override the default behavior of a 'get' returning a noncopyable type, so that it returns an owned value rather than a borrow, when that getter is exposed in opaque interfaces like protocol requirements or resilient types.

resolves rdar://157318147

[kavon]

@swift-ci smoke test

[nkcsgexi]

Will the @_owned attributes be printed into textual interfaces?

[kavon]

Yes, it is printed in the textual interface (see the SILGen/owned_attr.swift test). Properties with @_owned are #if-guarded by $UnderscoreOwned feature availability in the interface file. No -enable-experimental-feature UnderscoreOwned is required to use @_owned; compilers either know about it, or shouldn't see the property declaration in the interface.

[nkcsgexi]

Thank you for the context, Kavon! I think we should drop the UserInaccessible bit to leverage the existing infrastructure that guards attribute printing. Doug made a similar change recently for @_c in #86293.

[kavon]

@swift-ci smoke test

[kavon]

@swift-ci smoke test macOS