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_java.cpp

@@ -25,7 +25,9 @@
 namespace flatbuffers {
 namespace java {
 
-Namer::Config JavaDefaultConfig() {
+namespace {
+
+static Namer::Config JavaDefaultConfig() {
   return {
     /*types=*/Case::kKeep,
     /*constants=*/Case::kScreamingSnake,
@@ -50,7 +52,7 @@ Namer::Config JavaDefaultConfig() {
   };
 }
 
-std::set<std::string> JavaKeywords() {
+static std::set<std::string> JavaKeywords() {
   return {
     "abstract", "continue", "for",        "new",       "switch",
     "assert",   "default",  "goto",       "package",   "synchronized",
@@ -65,16 +67,18 @@ std::set<std::string> JavaKeywords() {
   };
 }
 
-static TypedFloatConstantGenerator JavaFloatGen("Double.", "Float.", "NaN",
+static const TypedFloatConstantGenerator JavaFloatGen("Double.", "Float.", "NaN",
                                                 "POSITIVE_INFINITY",
                                                 "NEGATIVE_INFINITY");
 
-static CommentConfig comment_config = {
+static const CommentConfig comment_config = {
   "/**",
   " *",
   " */",
 };
 
+} // namespace
+
 class JavaGenerator : public BaseGenerator {
   struct FieldArrayLength {
     std::string name;