Revert "Revert "PR14606: Debug info for using directives/DW_TAG_imported_module""

This reverts commit 179839 now that the corresponding LLVM patch has been fixed.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@179997 91177308-0d34-0410-b5e6-96231b3b80d8