Fix an incorrect 'Throws' in the regex code. Add a test for the new behavior. Reviewed as https://reviews.llvm.org/D61828. Thanks to Mark for the catch and the fix.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@361887 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed