GSB: Loosen up verifyGenericSignature()

We used to assert if a generic requirement of a signature could
be proved from the signature with the requirement removed.

However, in some rare cases minimization can *add* new requirements
as well as remove them.

This is going to come up even more with the RequirementMachine,
where for example the following signature is accepted:

    protocol Q { associatedtype A : P }
    protocol P { associatedtype B }
    <T where T.A : Q, T.A.B == T>

and minimized as follows:

    <T where T : P, T.A : Q, T.A.B == T>

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -8513,6 +8513,11 @@ void GenericSignatureBuilder::verifyGenericSignature(ASTContext &context,
       std::move(builder).computeGenericSignature(
                                       /*allowConcreteGenericParams=*/true);
 
+    // If the new signature once again contains the removed requirement, it's
+    // not redundant.
+    if (newSig->isEqual(sig))
+      continue;
+
     // If the removed requirement is satisfied by the new generic signature,
     // it is redundant. Complain.
     if (newSig->isRequirementSatisfied(requirements[victimIndex])) {