[libcxx] Make regex_match backtrack when search fails

Summary:
Fixes PR19851.
alg.re.match/ecma.pass.cpp still XFAILS on linux, but after commenting out
locale-related tests, it passes. I don't have a freebsd machine to produce a
full pass.

Reviewers: mclow.lists

Subscribers: cfe-commits, emaste

Differential Revision: https://reviews.llvm.org/D26026

llvm-svn: 285352