[libc++][NFC] Use concepts in <bit>.

All supported compilers have concepts support so use that in the C++20
functions in <bit>.

s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/ as drive-by fix.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D127594