This change implements the C library dependent portions of P0482R6
(char8_t: A type for UTF-8 characters and strings (Revision 6)) by
declaring std::c8rtomb() and std::mbrtoc8() in the <cuchar> header
when implementations are provided by the C library as specified by
WG14 N2653 (char8_t: A type for UTF-8 characters and strings
(Revision 1)) as adopted for C23.
A _LIBCPP_HAS_NO_C8RTOMB_MBRTOC8 macro is defined by the libc++ __config
header unless it is known that the C library provides these functions
in the current compilation mode. This macro is used for testing purposes
and may be of use to libc++ users. At present, the only C library known
to implement these functions is GNU libc as of its 2.36 release.
Reviewed By: ldionne
Differential Revision: https://reviews.llvm.org/D130946
Tom Honermann