ObjPtr<>-ify String allocations, fix stale refs.

ObjPtr<>-ify String allocation functions and related code
and remove some unnecessary calls to ObjPtr<>::Ptr(). Fix
stale reference uses in reference_table_test and stub_test.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: I42927fb8b7240e5132188f73318b2ccb218748fd
diff --git a/runtime/verifier/method_verifier.cc b/runtime/verifier/method_verifier.cc
index f32dd73..34b0f48 100644
--- a/runtime/verifier/method_verifier.cc
+++ b/runtime/verifier/method_verifier.cc
@@ -2724,7 +2724,7 @@
             : called_method->LookupResolvedReturnType();
         if (return_type_class != nullptr) {
           return_type = &FromClass(called_method->GetReturnTypeDescriptor(),
-                                   return_type_class.Ptr(),
+                                   return_type_class,
                                    return_type_class->CannotBeAssignedFromOtherTypes());
         } else {
           DCHECK(!can_load_classes_ || self_->IsExceptionPending());