More LWG issues. Mark #2182, #2323 and #2213 as complete. Add a test for #2339, and mark that as complete. No actual changes to the libc++ code; all of these were already in place.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202407 91177308-0d34-0410-b5e6-96231b3b80d8