PR18079: support nested autocast / @defined

PR18079: support nested autocast / @defined

We now perform const-folding & dead-code-elision during the type
resolution loop, whenever an autocast expression gets evaluated.  This
way, @defined(foo()->mm) type expressions can work as nature intended.

This requires @defined() not to be short-circuit evaluated to 0 during
a random const_folding process, so a flag is introduced to control its
preservation or collapsing.  For the last (assert_resolvability) pass
in the type resolution loop, this flag is set to true, so that
genuinely unresolvable @defined($expressions) do get mapped to 0 in
time for a last elision.