Revert "[Modules] Make header inclusion order from umbrella dirs deterministic"

Reverts commit r289478.

This broke
http://lab.llvm.org:8011/builders/clang-ppc64be-linux/builds/2070
(and maybe
http://lab.llvm.org:8011/builders/clang-x86_64-linux-selfhost-modules-2/builds/2246)

llvm-svn: 289494