[Bug c++/86974] Support Clang's require_constant_initialization attribute

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86974

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
libstdc++ sources do this:

#if __has_cpp_attribute(clang::require_constant_initialization)
#  define __constinit [[clang::require_constant_initialization]]
#endif

An alternative that doesn't use reserved names and so is suitable for user code
would be:

#if __cpp_constinit
# define CONSTINIT constinit
#if __has_cpp_attribute(clang::require_constant_initialization)
# define CONSTINIT [[clang::require_constant_initialization]]
#elif __GNUC__ >= 10
# define CONSTINIT __constinit
#else
# define CONSTINIT
#endif

And then:

CONSTINIT S2 objx;