[NFC][libc++][bitset] Refactors constructors.

Based on the review comments in D153201 this combines the string and
c-string constructors. The common constructor is using a string_view:
- it allows propagating the _Traits, which are required to be used for
  comparison.
- it avoids allocating
- libc++ supports it in C++03

Reviewed By: philnik, #libc, ldionne

Differential Revision: https://reviews.llvm.org/D154860