[dsymutil][DWARFv5] fix DW_FORM_addrx attribute offset calculation.

DWARFLinker::DIECloner::cloneAddressAttribute() contains call to
relocateIndexedAddr(StartOffset, EndOffset). StartOffset is
incorrectly calculated. Val.getRawUValue() is an index into the
.debug_addr table, so it should be multiplied
by Unit.getOrigUnit().getAddressByteSize().

Differential Revision: https://reviews.llvm.org/D132644