SIL: Simplify SILCloner::remapRootOpenedType()

Author: slavapestov

include/swift/SIL/SILCloner.h

@@ -254,13 +254,10 @@ class SILCloner : protected SILInstructionVisitor<ImplClass> {
     assert(archetypeTy->isRoot());
 
     auto sig = Builder.getFunction().getGenericSignature();
-    auto existentialTy = archetypeTy->getExistentialType()->getCanonicalType();
-    auto env = GenericEnvironment::forOpenedExistential(
-        getOpASTType(existentialTy), sig, UUID::fromTime());
-    auto interfaceTy = OpenedArchetypeType::getSelfInterfaceTypeFromContext(sig, existentialTy->getASTContext());
-    auto replacementTy =
-        env->mapTypeIntoContext(interfaceTy)
-            ->template castTo<OpenedArchetypeType>();
+    auto origExistentialTy = archetypeTy->getExistentialType()
+        ->getCanonicalType();
+    auto substExistentialTy = getOpASTType(origExistentialTy);
+    auto replacementTy = OpenedArchetypeType::get(substExistentialTy, sig);
     registerOpenedExistentialRemapping(archetypeTy, replacementTy);
   }