This fixes Mantis bug #485.
compiler/check_parse_tree_type_defns.m:
Generalize the representation of du types in checked type definitions
to give special representation to dummy types, as well as enum types.
Allow foreign enum definitions for types whose du definitions are dummy,
as well as those whose du definitions are enum.
compiler/prog_type.m:
Have the predicate that tests whether a du type is an enum type
to return the number of function symbols as well as the number of bits
needed to represent them. This is the most direct differentiator between
dummy types and enum types.
compiler/comp_unit_interface.m:
Conform to the change in prog_type.m.
tests/valid/bug485.m:
A regression test for the bug.
tests/valid/Mmakefile:
Enable the new test case.
