[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.

lib/Serialization/Serialization.cpp

@@ -5139,10 +5139,6 @@ public:
     llvm_unreachable("modules are currently not first-class values");
   }
 
-  void visitInverseType(const InverseType *) {
-    llvm_unreachable("inverse types should not escape the type checker");
-  }
-
   void visitInOutType(const InOutType *) {
     llvm_unreachable("inout types are only used in function type parameters");
   }