AST: Use ArchetypeType::getExistentialType() in a couple of places

Author: slavapestov

lib/AST/Type.cpp

@@ -645,10 +645,7 @@ Type TypeBase::typeEraseOpenedArchetypesWithRoot(
         for (auto replacementType :
                  opaque->getSubstitutions().getReplacementTypes()) {
           if (replacementType->hasOpenedExistentialWithRoot(root)) {
-            Type interfaceType = opaque->getInterfaceType();
-            auto genericSig =
-                opaque->getDecl()->getOpaqueInterfaceGenericSignature();
-            return genericSig->getNonDependentUpperBounds(interfaceType);
+            return opaque->getExistentialType();
           }
         }
       }

lib/Sema/ConstraintSystem.cpp

@@ -2174,10 +2174,7 @@ static Type typeEraseExistentialSelfReferences(Type refTy, Type baseTy,
             for (auto replacementType :
                  opaque->getSubstitutions().getReplacementTypes()) {
               if (hasErasedGenericParameter(replacementType)) {
-                Type interfaceType = opaque->getInterfaceType();
-                auto genericSig =
-                    opaque->getDecl()->getOpaqueInterfaceGenericSignature();
-                return genericSig->getNonDependentUpperBounds(interfaceType);
+                return opaque->getExistentialType();
               }
             }
           }