[llvm][ADT] Implement `BitVector::{pop_,}back`

LLVM Programmer’s Manual strongly discourages the use of `std::vector<bool>` and suggests `llvm::BitVector` as a possible replacement.

Currently, some users of `std::vector<bool>` cannot switch to `llvm::BitVector` because it doesn't implement the `pop_back()` and `back()` functions.

To enable easy transition of `std::vector<bool>` users, this patch implements `llvm::BitVector::pop_back()` and `llvm::BitVector::back()`.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D117115