[lib-c++] Bug 42199 - Wrong implementation for match_prev_avail

Hi,

I have a small patch stalled for fixing a libc++ bug when using the flag
regex_constants::match_prev_avail.
By the specification, it's use should ignore the flags match_not_bol |
match_not_bow.

https://reviews.llvm.org/D75622

Any comments would be appreciated.

Regards

Diogo

(moving to libcxx-dev)