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/idl_gen_go.cpp

@@ -38,8 +38,10 @@ namespace flatbuffers {
 
 namespace go {
 
+namespace {
+
 // see https://golang.org/ref/spec#Keywords
-std::set<std::string> GoKeywords() {
+static std::set<std::string> GoKeywords() {
   return {
     "break",    "default",     "func",   "interface", "select",
     "case",     "defer",       "go",     "map",       "struct",
@@ -49,7 +51,7 @@ std::set<std::string> GoKeywords() {
   };
 }
 
-Namer::Config GoDefaultConfig() {
+static Namer::Config GoDefaultConfig() {
   // Note that the functions with user defined types in the name use
   // upper camel case for all but the user defined type itself, which is keep
   // cased. Despite being a function, we interpret it as a Type.
@@ -75,6 +77,8 @@ Namer::Config GoDefaultConfig() {
            /*filename_extension=*/".go" };
 }
 
+} // namespace
+
 class GoGenerator : public BaseGenerator {
  public:
   GoGenerator(const Parser &parser, const std::string &path,