Improve accessibility in debugging for free functions

[mingxwa]

Motivation

Similar to #200, implemented a mechanism to guide compilers to generate code for unused free functions to facilitate debugging.

Changes

• For each built-in dispatch type that generates a free function (including operators) for accessibility, enabled symbol generation by creating a private static function _symbol_guard in its accessor type to invoke the free function via ADL.

Verification

Build the following code with MSVC in debug mode (without using the value of proxy in the code):

#include <string>

#include "proxy.h"

PRO_DEF_FREE_DISPATCH(FreeToString, std::to_string, ToString);

struct Stringable : pro::facade_builder
  ::add_convention<FreeToString, std::string() const>
  ::build {};

int main() {
  int v = 123;
  pro::proxy<Stringable> p = &v;
}

Set a breakpoint at the end of main and inspect the value of p in the debugger. The debugger invokes the correct function for me.