Apply LWG 2048.  It is amazing to me that this actually works, but the existing tests confirm that it does.  c++1y status page now showing libc++ is complete for c++1y modulo dynarray issues.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@191142 91177308-0d34-0410-b5e6-96231b3b80d8