[llvm] Move bit counting functions to bit.h (NFC)

This patch provides C++20-style countl_zero, countr_zero, countl_one,
and countr_one in bit.h.  Existing functions like countLeadingZeros
become wrappers around the new functions.

Note that I cannot quite declare countLeadingZeros as:

  template <class T> using countLeadingZeros = countl_zero<T>;

because countl_zero returns int, whereas countLeadingZeros returns
unsigned.

Differential Revision: https://reviews.llvm.org/D142078