[libcxx] Implement P1956 rename low-level bit functions

Implements P1956: On the names of low-level bit manipulation functions.

Users may use older versions of libc++ or other standard libraries with the old names. In order to keep compatibility the old functions are kept, but marked as deprecated.

The patch also adds a new config macro `_LIBCPP_DEPRECATED_MSG`. Do you prefer a this is a separate patch?

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D90551