[libc++] Use unsigned char in basic_string::__short again

D125496 changed the string layout on windows. Change `__is_long_` and `__size_` back to using `unsigned char` to fix the issue.

Reviewed By: Mordante, #libc

Spies: jloser, libcxx-commits, ayzhao

Differential Revision: https://reviews.llvm.org/D127566