[Bug c++/128116] [14/15/16/17 Regression] static constexpr reference can bind to nullptr while inside a template

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128116

--- Comment #2 from Drea Pinski <pinskia at gcc dot gnu.org> ---
Note outside of a template, GCC correctly rejects the code.  That is this is
rejected:
```
struct S1
{
    static constexpr int &foo = 0;
};
```