[FIX][libc++][Regex] Using regex_constants match_prev_avail | match_not_bol | match_not_bow
When using regex_constants::match_prev_avail, it is defined that
--first is valid, and match_not_bol and match_not_bow should be
ignored. At the moment these flags are not ignored. This fixis that.
Reviewers: ldionne, miyuki, EricWF, mclow.lists, #libc
Reviewed By: ldionne, miyuki, #libc
Subscribers: broadwaylamb, dexonsmith, libcxx-commits
Differential Revision: https://reviews.llvm.org/D75622