[ADT] Provide C++20-style bit functions

Tihs patches adds APInt::count{l,r}_{zero,one} and APInt::popcount to
be consistent with those functions in ADT/bit.h.

Once this patch lands, I'll take care of the migration.

For now, I am intentionally leaving isPowerOf2 as is.

Differential Revision: https://reviews.llvm.org/D144165