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/bfbs_gen.h

@@ -24,19 +24,21 @@
 
 namespace flatbuffers {
 
-void ForAllEnums(
+namespace {
+
+static void ForAllEnums(
     const flatbuffers::Vector<flatbuffers::Offset<reflection::Enum>> *enums,
     std::function<void(const reflection::Enum *)> func) {
   for (auto it = enums->cbegin(); it != enums->cend(); ++it) { func(*it); }
 }
 
-void ForAllObjects(
+static void ForAllObjects(
     const flatbuffers::Vector<flatbuffers::Offset<reflection::Object>> *objects,
     std::function<void(const reflection::Object *)> func) {
   for (auto it = objects->cbegin(); it != objects->cend(); ++it) { func(*it); }
 }
 
-void ForAllEnumValues(const reflection::Enum *enum_def,
+static void ForAllEnumValues(const reflection::Enum *enum_def,
                       std::function<void(const reflection::EnumVal *)> func) {
   for (auto it = enum_def->values()->cbegin(); it != enum_def->values()->cend();
        ++it) {
@@ -44,7 +46,7 @@ void ForAllEnumValues(const reflection::Enum *enum_def,
   }
 }
 
-void ForAllDocumentation(
+static void ForAllDocumentation(
     const flatbuffers::Vector<flatbuffers::Offset<flatbuffers::String>>
         *documentation,
     std::function<void(const flatbuffers::String *)> func) {
@@ -89,6 +91,8 @@ static bool IsVector(const reflection::BaseType base_type) {
   return base_type == reflection::Vector;
 }
 
+} // namespace
+
 // A concrete base Flatbuffer Generator that specific language generators can
 // derive from.
 class BaseBfbsGenerator : public BfbsGenerator {
@@ -187,4 +191,4 @@ class BaseBfbsGenerator : public BfbsGenerator {
 
 }  // namespace flatbuffers
 
-#endif  // FLATBUFFERS_BFBS_GEN_H_
+#endif  // FLATBUFFERS_BFBS_GEN_H_