Bill Fisher:  Fix for failing to throw an exception in regex when parsing an invalid escape sequence.  This fixes http://llvm.org/bugs/show_bug.cgi?id=16023

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185192 91177308-0d34-0410-b5e6-96231b3b80d8