Address review feedback for r212238.

Also, forgot to say in the commit message of r212238: Library authors will
see a warning about this issue if they build with -Wsystem-headers.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212243 91177308-0d34-0410-b5e6-96231b3b80d8