RequirementMachine: fix allowInverses handling

We were skipping the expansion of default requirements on generic
parameters when creating an abstract generic signature that adds only
new generic parameters, but no additional requirements.

Author: kavon

lib/AST/RequirementMachine/RequirementMachineRequests.cpp

@@ -565,7 +565,7 @@ AbstractGenericSignatureRequest::evaluate(
 
   // If there are no added requirements, we can form the signature directly
   // with the added parameters.
-  if (addedRequirements.empty()) {
+  if (addedRequirements.empty() && !allowInverses) {
     auto result = GenericSignature::get(genericParams,
                                         baseSignature.getRequirements());
     return GenericSignatureWithError(result, GenericSignatureErrors());