Add dwarf2_per_cu_data::addresses_seen

Add dwarf2_per_cu_data::addresses_seen

This adds a new member to dwarf2_per_cu_data that indicates whether
addresses have been seen for this CU.  This is then set by the
.debug_aranges reader.  The idea here is to detect when a CU does not
have address information, so that the new indexer will know to do
extra scanning in that case.