Sema: Only diagnose explicit unavailable Clang enum elements.
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.
