Revert "[modules] Fix macro hiding bug exposed if:"

This is breaking the system modules on Darwin, because something that
was defined and re-exported no longer is.  Might be this patch, or might
just be a really poor interaction with an existing visibility bug.

This reverts commit r213348.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213395 91177308-0d34-0410-b5e6-96231b3b80d8