Extensions that extend non-public types should never be required to have
explicit availability, even if they declare conformances to public protocols.
Resolves rdar://148697770.
The changes in https://github.com/swiftlang/swift/pull/80040 caused the
compiler to start diagnosing extensions containing only members that are
either `@_spi`, `@_alwaysEmitIntoClient`, or unavailable when the
`-require-explicit-availability` flag is passed. Extensions should not be
diagnosed when they only contain members that would not be diagnosed
themselves.
Resolves rdar://148275432.
Extending a protocol cannot introduce new conformances to other protocols, so
skip checking for associated conformances to public protocols when diagnosing
required availability on such an extension.
Resolves rdar://133873836.
