[libcxx] Correctly handle invalid regex character class names

Summary:
Currently when a regular expression contains an invalid character
class name std::regex constructors throw an std::regex_error with
std::regex_constants::error_brack code.

This patch changes the code to std::regex_constants::error_ctype and
adds a test.

Reviewers: EricWF, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42291

llvm-svn: 323322