Replace a dummy pred_id in a trace goal.

compiler/polymorphism.m:
	Replace a dummy pred_id in a trace goal to that which was
	intended, judging by the preceding comment.
Fix inst for constant type_infos.

The inst for type_infos entered into the const_struct database did not
count the type_ctor_info argument, thus the cons_id arity was off by one.
Bug #297.

compiler/polymorphism.m:
        Account for the type_ctor_info when making the inst for a constant
        type_info.

tests/hard_coded/Mercury.options:
tests/hard_coded/Mmakefile:
tests/hard_coded/type_info_const_inst.exp:
tests/hard_coded/type_info_const_inst.m:
        Add test case.

NEWS:
        Announce the change.