[libc++] [LWG3656] Update the return type of std::bit_width.

Fixes LWG3656, "Inconsistent bit operations returning a count".
https://cplusplus.github.io/LWG/issue3656

The fix has been approved for C++23 and left to vendors' discretion
in C++20 (but it sounds like everyone's on the same page that
of course it should be DR'ed back to C++20 too).

Differential Revision: https://reviews.llvm.org/D120444