[ADT] Add bit_width_constexpr (#161775)

This patch adds llvm::bit_width_constexpr, a constexpr version of
llvm::bit_width.

The new function is intended to serve as a marker.  When we switch to
C++20, we will most likely go through functions in llvm/ADT/bit.h and
replace them with their counterparts from <bit>.  With
llvm::bit_width_constexpr, we can easily replace its use with
std::bit_width.

This patch refactors a couple of places.  Specifically:

- bitWidth in BitmaskEnum.h is replaced with the new function.

- bitsRequired in PointerUnion.h is redefined in terms of the new
  function.

I've used Compiler Explorer to check the equivalence:

https://godbolt.org/z/1oKMK9Ez7

0 files changed, 0 insertions, 0 deletions