[libc++] Adds __utility/to_underlying.h.

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