[libcxx] Support std::regex_constants::match_not_null

Summary: Fixes PR21597.

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25595

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@284881 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed