6.0: [TypeCheckAttr] @frozen is valid even when building without enable-library-evolution

[nate-chandler]

Explanation: Fix inference of Sendable/BitwiseCopyable on @frozen enums in non-resilient modules like the embedded stdlib.

Previously, when type-checking the @frozen attribute on an enum, if the module was being built non-resiliently (without -enable-library-evolution), the attribute would silently be invalidated and ignored. It was ignored even if the attribute was applied to a non-exported type.

Inference of conformance to Sendable and BitwiseCopyable only proceeds on exported types if they are marked @frozen (with a non-invalidated attribute). The silent invalidation of @frozen on exported enums in non-resilient modules resulted in conformance not being inferred for those enums. For example, the embedded stdlib is built without resilience, so neither UnboundedRange_'s conformance to Sendable nor Never's conformance to BitwiseCopyable were inferred.

Here, this is fixed. Exported enums keep the attribute, and non-exported enums get a warning if the attribute is applied.

Fixes a failure to infer conformances to Sendable and BitwiseCopyable in the stdlib when building for embedded.
Scope: Affects @frozen annotations.
Issue: rdar://128358780
Original PR: #73756
Risk: Low. Does unsuppress a warning when using @frozen on non-exported enums in non-resilient modules, however.
Testing: Added a test demonstrating that Sendable/BitwiseCopyable inference on @frozen exported enums in non-resilient modules occurs as intended.
Reviewer: Doug Gregor ( @DougGregor )

[nate-chandler]

@swift-ci please test

[nate-chandler]

@swift-ci please test source compatibility