In cases where the generic parameter is class-constrained,
`GenericSignature::requiresProtocol` will not contain `Copyable` or
`Escapable` because GenericSignature minimization will recognize that
the class already requires them.
Thus, because classes always require those protocols, we can
simply ask if the generic parameter is required to be a class to
determine if it had any inverses.
We can't simply emit the desugared, expanded version of the requirements
because there's no way to pretty-print the type `some ~Copyable` when
the `~Copyable`'s get replaced with the absence of `Copyable`. We'd be
left with just `some _` or need to invent a new top type so we can write
`some Top`. Thus, it's best to simply reverse the expansion of default
requirements when emitting a swiftinterface file.
