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/test_builder.cpp

@@ -117,6 +117,9 @@ bool release_n_verify(flatbuffers::FlatBufferBuilder &fbb,
   return verify(buf, expected_name, color);
 }
 
+// forward-declared in test.cpp
+void FlatBufferBuilderTest();
+
 void FlatBufferBuilderTest() {
   using flatbuffers::FlatBufferBuilder;
 
@@ -138,5 +141,8 @@ void FlatBufferBuilderTest() {
       TestSelector(tests, tests + 4));
 }
 
+// forward-declared in test_builder.h
+void CheckTestGeneratedIsValid(const MyGame::Example::Color&);
+
 // Link-time check using pointer type.
 void CheckTestGeneratedIsValid(const MyGame::Example::Color &) {}