Undo include order work-around in Regex.cpp

r327256 / 2b8b90a7 added this
include-order work-around since regex_impl.h's include guard conflicted
with macOS's xlocale.h which used the same macro.

Since b86c2496 renamed the include
guard, that work-around is no longer needed.

Differential revision: https://reviews.llvm.org/D150127