Implement LWG2784, and mark 2786, 2795, 2804, 2812, 2826, 2834, 2837 and 2838 as complete - since we do them already

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@297752 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed