We've stopped doing it in libc++ for a while now because these names
would end up rotting as we move things around and copy/paste stuff.
This cleans up all the existing files so as to stop the spreading
as people copy-paste headers around.
During the review of D97115 it was mentioned adding the `<utility>`
header for `__to_underlying` was a bit unfortunate. Nowadays we tend to
implement smaller headers, so a good reason to move `std::to_underlying`
to its own header and adjust `<charconv>` to use the new header.
Differential Revision: https://reviews.llvm.org/D101233
Arthur O'Dwyer