[libc++] Use bit field for checking if string is in long or short mode

This makes the code a bit simpler and (I think) removes the undefined behaviour from the normal string layout.

Reviewed By: ldionne, Mordante, #libc

Spies: labath, dblaikie, JDevlieghere, krytarowski, jgorbe, jingham, saugustine, arichardson, libcxx-commits

Differential Revision: https://reviews.llvm.org/D123580