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.

src/flatc.cpp

@@ -25,7 +25,7 @@
 
 namespace flatbuffers {
 
-const char *FLATC_VERSION() { return FLATBUFFERS_VERSION(); }
+static const char *FLATC_VERSION() { return FLATBUFFERS_VERSION(); }
 
 void FlatCompiler::ParseFile(
     flatbuffers::Parser &parser, const std::string &filename,