[m-rev.] for review: Fix MR_named_arg_num for RTTI changes introduced with subtypes.

[Julien Fischer]

On Wed, 7 Apr 2021, Peter Wang wrote:

> runtime/mercury_deconstruct.c:
>    Use MR_index_or_search_ptag_layout to search du type layouts by
>    primary tag and MR_index_or_search_sectag_functor to search
>    sectag_alternatives by secondary tag, as required for subtypes.
>
>    (MR_named_arg_num is used by the debugger.)

That's fine.

Julien.