[ADT] Add bit_floor, bit_ceil, and bit_width to bit.h

This patch adds C++20-style bit_floor, bit_ceil, and bit_width.

In a subsequent patch, I'm going to define PowerOf2Floor in
MathExtras.h in terms of bit_floor.

Unfortunately, PowerOf2Ceil isn't quite the same as bit_ceil because
PowerOf2Ceil(0) == 0, whereas bit_ceil(0) == 1.

MathExtras.h does not have a function directly corresponding to
bit_width, but Log2_32(X) + 1, which occurs in a few places, can be
replaced with bit_width(X).

Differential Revision: https://reviews.llvm.org/D142179