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	for DW_FORM_ref_addr, handle DW_FORM_sec_offset, DW_FORM_exprloc
	and DW_FORM_flag_present.  For unknown form value return NULL.
	(scan_unit_for_symbols): For DW_AT_location handle DW_FORM_exprloc
	like DW_FORM_block.
	(parse_comp_unit): Allow CU version 4.