[C++] Add ParseJson(), Parser(Parser&&), update fuzzers (#6284)

- add a new method ParseJson to minimize failures during fuzzing
- add default (conditional) move-constructor for Parser
- add a new monster_fuzzer
- switch fuzzers to C++17 and `test/cpp17` generated code

tests/fuzzer/flatbuffers_scalar_fuzzer.cc

@@ -1,6 +1,23 @@
+/*
+ * Copyright 2014 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 #include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
+
 #include <algorithm>
 #include <clocale>
 #include <memory>
@@ -196,7 +213,7 @@ class ScalarReferenceResult {
 
 bool Parse(flatbuffers::Parser &parser, const std::string &json,
            std::string *_text) {
-  auto done = parser.Parse(json.c_str());
+  auto done = parser.ParseJson(json.c_str());
   if (done) {
     TEST_EQ(GenerateText(parser, parser.builder_.GetBufferPointer(), _text),
             true);