[m-dev.] For review: Fix a bug in the external debugger

[Erwan Jahier]

| The actual comment in mercury_trace_layout.c is:
| 
| /*
| ** get_type_and_value() and get_type() will succeed to retrieve "variables"
| ** that we do not want to send to the user; "variables" beginning with
| ** `ModuleInfo', or `HLDS' may occur when debugging the compiler and are too
| ** big to be displayed.  "Variables" beginning with `TypeInfo' denote the
| ** additional parameters introduced by compiler/polymorphism.m that we don't
| ** want to show neither.
| ** That's why we define filtered version of get_type_and_value() and get_type()
| ** that will fail to retrieve such variables.
| */
| 
| Shouldn't it be (as in mercury-trace_internal.c):
| 
| /*
| ** XXX The printing of type_infos is buggy at the moment
| ** due to the fake arity of the type private_builtin:typeinfo/1.
| */
| 
| ?
| Or do I miss something ?

I will replace it by 

/*
** get_type_and_value() and get_type() will succeed to retrieve "variables"
** beginning with `TypeInfo' and `TypeClassInfo'. As we can not print those
** variables because of the fake arity of the type private_builtin:typeinfo/1,
** we define filtered version of get_type_and_value() and get_type()
** that will fail to retrieve such variables.
*/

and add 

/*
** XXX The printing of type_infos is buggy at the moment
** due to the fake arity of the type private_builtin:typeinfo/1.
*/

in mercury_trace-external.c and commit that change.
-- 
R1.