AST: Fix invariant violation in swift::validateGenericSignature()

This is a silly "extended" verification check that we only exercise
in test/Generics/validate_stdlib_generic_signatures.swift.

We were taking a requirement containing unbound dependent member types
and applying a substitution map to it, which would result in an
ErrorType, so the requirement was always considered unsatisfied here
and the check was not as useful as it should have been.

Instead, re-implement a version of isRequirementSatisfied() that only
uses generic signature queries instead of substitution here.

Author: slavapestov

lib/AST/GenericSignature.cpp

@@ -1174,7 +1174,39 @@ void swift::validateGenericSignature(ASTContext &context,
 
     // If the removed requirement is satisfied by the new generic signature,
     // it is redundant. Complain.
-    if (newSig->isRequirementSatisfied(requirements[victimIndex])) {
+    auto satisfied = [&](Requirement victim) {
+      if (!newSig->isValidTypeParameter(victim.getFirstType()))
+        return false;
+
+      switch (victim.getKind()) {
+      case RequirementKind::SameShape:
+        return (newSig->isValidTypeParameter(victim.getSecondType()) &&
+                newSig->haveSameShape(victim.getFirstType(),
+                                      victim.getSecondType()));
+      case RequirementKind::Conformance:
+        return newSig->requiresProtocol(victim.getFirstType(),
+                                        victim.getProtocolDecl());
+      case RequirementKind::Superclass: {
+        auto superclass = newSig->getSuperclassBound(victim.getFirstType());
+        return (superclass && superclass->isEqual(victim.getSecondType()));
+      }
+      case RequirementKind::SameType:
+        if (!victim.getSecondType().findIf([&](Type t) -> bool {
+            return (!t->isTypeParameter() ||
+                    newSig->isValidTypeParameter(t));
+          })) {
+          return false;
+        }
+        return newSig.getReducedType(victim.getFirstType())
+          ->isEqual(newSig.getReducedType(victim.getSecondType()));
+      case RequirementKind::Layout: {
+        auto layout = newSig->getLayoutConstraint(victim.getFirstType());
+        return (layout && layout == victim.getLayoutConstraint());
+      }
+      }
+    };
+
+    if (satisfied(requirements[victimIndex])) {
       SmallString<32> reqString;
       {
         llvm::raw_svector_ostream out(reqString);