Mark LWG#2260 as complete. We already did the right thing, so I just added...

Mark LWG#2260 as complete. We already did the right thing, so I just added tests to ensure that we continue to DTRT.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@338936 91177308-0d34-0410-b5e6-96231b3b80d8