https://github.com/swiftlang/swift/pull/77236 caused a source compatibility
regression because `extractEnumElement()` does not suppress its diagnostics in
the context of pattern matching. Potentially unavailable enum elements should
not be diagnosed when pattern matching since the generated code will not
retrieve the potentially unavailable element value on versions where it is
unavailable.
Fixes rdar://138771328.
The current implementation of `-application-extension` has a problem that affects the generation of ObjC headers for regular Swift modules.
The primary purpose of `-application-extension` is to prevent the use of unavailable APIs in app extensions. However, it has an impact on the generation of -Swift.h headers and exposes Swift's internal declarations to ObjC. This behavior is appropriate for mixed modules that are not consumed externally, such as app extensions, but it fails to address the situation when a module is not an extension itself but is consumed by the extension (c90cd11).
To resolve this issue while maintaining the desired behavior, we can introduce a new flag for this particular use-case.
