[Bug libstdc++/122905] [16 regression] ODR warning with C++11 and C++20 std::string use

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122905

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
a.cc:
```
#include "a.h"

std::string example()
{
    return "TEST";
}
```
a.h:
```
#ifndef A_H
#define A_H

#include<string>

std::string example();

#endif
```
b.cc
```
#include "a.h"

auto foo = example();
```