[libcxx] basic_regex: add traits_type, string_type

Summary:
In the synopsis in C++11 subclause 28.8 [re.regex], `basic_regex` is
specified to have member typedefs `traits_type` and `string_type`. This
change adds them to libc++.

Reviewers: mclow.lists, rsmith, hubert.reinterpretcast

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D22698

Patch by Jason Liu!

llvm-svn: 277526