[FIX][libc++][Regex] Using regex_constants match_prev_avail | match_not_bol | match_not_bow

Summary:
pr42199
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

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75622