[NCGenerics] fold InverseType into PCT

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.

test/Generics/inverse_protocols_errors.swift

@@ -3,8 +3,8 @@
 // REQUIRES: asserts
 
 protocol RegularProto {}
-protocol NCProto: ~Copyable, RegularProto {
-  // expected-warning@-1 {{protocol 'NCProto' should be declared to refine 'Copyable' due to a same-type constraint on 'Self'}}
+protocol NCProto: RegularProto
+  where Self: ~Copyable { // expected-error{{'Self' required to be 'Copyable' but is marked with '~Copyable'}}
   func checkIfCopyableSelf(_ s: Self)
 }