Sema: Extend exportability checking for non-library-evolution to more reasons

[xymus]

Extend exportability checking in non-library-evolution mode to handle more reasons (the classification of why references are restricted) to prepare for enabling this check as warning by default. Notably add a custom diagnostics for the use of restricted references in enum associated values and in conformance in non-implementation-only types.

Allow the use of @_implementationOnly on types without enabling the experimental feature. This is to prepare removing the feature flag.

[xymus]

@swift-ci Please smoke test