Sema: Loosen more available than enclosing extension diagnostic.
When diagnosing a declaration that is more available than its context, to
preserve source compatibility we need to downgrade the diagnostic to a warning
when the outermost declaration is an extension. This logic regressed with
https://github.com/swiftlang/swift/pull/77950 and my earlier attempt to fix
this (https://github.com/swiftlang/swift/pull/78832) misidentified what had
regressed.
Really resolves rdar://143423070.
