Fix comment that was obsoleted by r216949

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216999 91177308-0d34-0410-b5e6-96231b3b80d8
Fix yet another aspect of the build breakage caused by r216949

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217001 91177308-0d34-0410-b5e6-96231b3b80d8