When a NoncopyableGenericsMismatch happens between the compiler and
stdlib, allow the compiler to rebuild the stdlib from its interface
instead of exiting with an error.
We already need to track the inverses separate from the members in a
ProtocolCompositionType, since inverses aren't real types. Thus, the
only purpose being served by InverseType is to be eliminated by
RequirementLowering when it appears in a conformance requirement.
Instead, we introduce separate type InverseRequirement just to keep
track of which inverses we encounter to facilitate cancelling-out
defaults and ensuring that the inverses are respected after running
the RequirementMachine.
We already don't diagnose all redundant requirements. Because inverses
can be written in both inheritance and where clauses, but they're not
treated uniformly in the implementation, it's a bit annoying to try and
account for the redundancies in both places; `checkInheritanceClause`
will go over the same "requirements" that we'll also check again in
`swift::rewriting::expandDefaultRequirements`.
This implementation has the function execute a request to scan the
inheritance clause of non-protocol nominals for a `~Copyable`. For
protocols, we look in the requirement signature.
This isn't our final state, as the GenericEnvironment needs to be
queried in general to determine of a Type is noncopyable. So for now
checking for a `~Copyable` only makes sense for Decls.
