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.

grpc/src/compiler/python_generator.cc

@@ -23,8 +23,9 @@
 #include "src/compiler/python_generator.h"
 
 namespace grpc_python_generator {
+namespace {
 
-grpc::string GenerateMethodType(const grpc_generator::Method *method) {
+static grpc::string GenerateMethodType(const grpc_generator::Method *method) {
 
   if (method->NoStreaming())
     return "unary_unary";
@@ -131,6 +132,7 @@ void GenerateRegister(const grpc_generator::Service *service,
   printer->Outdent();
   printer->Print("\n");
 }
+} // namespace
 
 grpc::string Generate(grpc_generator::File *file,
                       const grpc_generator::Service *service) {