[libc++] Simplify single-iterator __bit_iterator algorithm specializations

The single-iterator algorithms have an implementation for `false` and `true`, which are almost identical. Instead of writing two functions, this refactors the code to take the value searched for as a template parameter. This avoids a lot of code duplication and makes it easier to reason about the algorithm and their difference.

Reviewed By: #libc, Mordante

Spies: Mordante, libcxx-commits

Differential Revision: https://reviews.llvm.org/D156033