for review: fix for inlining of polymorphic procedures

[Simon Taylor]

> If this might occur in user code (as opposed to compiler generated
> intermediate code), I guess you'd better to a Prolog version in
> the library so that it works in Sicstus.

Use of unsafe_type_cast/2 in user code is strongly discouraged, which
is why the declaration is in the section of the interface that is not
included in the documentation.

>From mercury_builtin.m:
        % unsafe_type_cast/2 is used internally by the compiler. Bad things
        % will happen if this is used in programs.

Among the bad things that will happen is that it won't work in Prolog.

Simon.