Fix compilation on Darwin with expensive checks.

After r327219 was landed, the bot with expensive checks on GreenDragon
started failing. The problem was missing symbols `regex_t` and
`regmatch_t` in `xlocale/_regex.h`. The latter was included because
after the change in r327219, `random` is needed, which transitively
includes `xlocale.h.` which in turn conditionally includes
`xlocale/_regex.h` when _REGEX_H_ is defined. Because this is the header
guard in `regex_impl.h` and because `regex_impl.h` was included before
the other LLVM includes, `xlocale/_regex.h` was included without the
necessary types being available.

This commit fixes this by moving the include of `regex_impl.h` all the
way down. I also added a comment to stress the significance of its
position.

llvm-svn: 327256