[PATCH RESEND 1/1] p1689r5: initial support

[Ben Boeckel]

On Thu, Oct 20, 2022 at 11:39:25 -0400, Jason Merrill wrote:
> Oops, I was thinking this was in gcc as well.  In libcpp there's 
> _cpp_valid_utf8 (which calls one_utf8_to_cppchar).

This routine has a lot more logic (including UCN decoding) and the
`one_utf8_to_cppchar` also supports out-of-bounds codepoints above
`0x10FFFF`.

--Ben