Audit and fixups for GCC and Clang (#7212)

Added (for compiler versions that support it):
-Wmissing-declarations
-Wzero-as-null-pointer-constant

Then, fixes to problems identified by the extra warnings
Tested only on GCC 9.4.0

Adjusted the CPP code generator to output nullptr where appropriate,
to satisfy -Wzero-as-null-pointer-constant

Added a lot of 'static' declarations in front of functions,
to satisfy -Wmissing-declarations,
and wrap static function defs in anonymous namespaces.

There are advantages to both anonymous namespaces and static,
it seems that marking a function as static will not publish the name in
the symbol table at all, thus giving the linker less work to do.

tests/cpp17/test_cpp17.cpp

@@ -47,6 +47,7 @@ using ::cpp17::MyGame::Example::Vec3;
 /*******************************************************************************
 ** Build some FB objects.
 *******************************************************************************/
+namespace {
 const Monster *BuildMonster(flatbuffers::FlatBufferBuilder &fbb) {
   using ::cpp17::MyGame::Example::Color;
   using ::cpp17::MyGame::Example::MonsterBuilder;
@@ -250,6 +251,7 @@ int FlatBufferCpp17Tests() {
   StringifyAnyFlatbuffersTypeTest();
   return 0;
 }
+} // namespace
 
 int main(int /*argc*/, const char * /*argv*/[]) {
   InitTestEngine();