Debug Info: use DW_FORM_ref_addr instead of DW_FORM_ref4 if the referenced DIE
belongs to a different compile unit.

DW_FORM_ref_addr should be used for cross compile-unit reference.

When compiling a large application, we got a dwarfdump verification error where
abstract_origin points to nowhere.

This error can't be reproduced on any testing case in MultiSource.
We may have other cases where we use DW_FORM_ref4 unconditionally.

rdar://problem/13370501