Implement LWG #2212: std::is_final. This requires compiler support, which modern versions of clang provide. Also mark LWG #2230 as complete - no code changes needed.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202934 91177308-0d34-0410-b5e6-96231b3b80d8