There's no good reason to permit them. Conformances like Copyable and
Sendable are pervasive, so it's as though we are permitting extensions
of `Any`. Until there's a good argument in favor of such extensions,
remove the capability now.
Some places use `getDirectlyInheritedNominalTypeDecls` directly
instead of fetching it through a request, so lift the check there
instead (direct use of `getDirectlyInheritedNominalTypeDecls`
could be refactored separately).
ClangImporter converts NS_SWIFT_SENDABLE into `SynthesizedProtocolAttr`
which needs to be recognized by the request that collects all of the
protocols that the given protocol inherits.
