[CMake] : Add precompiled header support with FLATBUFFERS_ENABLE_PCH (#5827)

* [CMake] : Add precompiled header support with FLATBUFFERS_ENABLE_PCH

FLATBUFFERS_ENABLE_PCH enable precompile headers support for 'flatbuffers' and 'flatc'. Default if OFF.
You need to set it to ON to make it work. 'cmake -DFLATBUFFERS_ENABLE_PCH=ON ..'
This will only work if CMake version is >= 3.16, because target_precompile_headers is used.
If CMake doesn't have target_precompile_headers, then nothing will happen.
This can speed up compilation time.
Precompiled header file for 'flatbuffers' is 'pch.h'
Precompiled header file for 'flatc' is 'flatc_pch.h'

* Enable FLATBUFFERS_ENABLE_PCH for VC 2017 and VS2019 builds

* [CMake]: Fix error in FLATBUFFERS_ENABLE_PCH description

* Add a function add_pch_to_target to avoid copy and pasting the same code like add_fsanitize_to_target
Move pch.h and flatc_pch.h from include/flatbuffers to include/flatbuffers/pch
Make flatc_pch.h depends on pch.h since it extend it

include/flatbuffers/pch/flatc_pch.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2017 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.
+ */
+
+#ifndef FLATBUFFERS_FLATC_PCH_H_
+#define FLATBUFFERS_FLATC_PCH_H_
+
+// stl
+#include <cmath>
+#include <sstream>
+#include <cassert>
+#include <unordered_set>
+#include <unordered_map>
+#include <iostream>
+#include <functional>
+#include <set>
+#include <iterator>
+#include <tuple>
+
+// flatbuffers
+#include "flatbuffers/pch/pch.h"
+#include "flatbuffers/code_generators.h"
+#include "flatbuffers/flatbuffers.h"
+#include "flatbuffers/flexbuffers.h"
+#include "flatbuffers/idl.h"
+
+#endif // FLATBUFFERS_FLATC_PCH_H_

include/flatbuffers/pch/pch.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2017 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.
+ */
+
+#ifndef FLATBUFFERS_PCH_H_
+#define FLATBUFFERS_PCH_H_
+
+// stl
+#include <cstdint>
+#include <cstring>
+#include <algorithm>
+#include <list>
+#include <string>
+#include <utility>
+#include <iomanip>
+#include <map>
+#include <memory>
+#include <limits>
+#include <stack>
+#include <vector>
+#include <type_traits>
+
+// flatbuffers
+#include "flatbuffers/util.h"
+
+#endif // FLATBUFFERS_PCH_H_