Use Substitution::subst() to replace the opened existential with
the concrete type. I don't have a test case, but I think the old
code would have failed if a non-Self substitution also contained
the opened existential, which could happen after generic inlining.
Also, it looks like the guard against devirtualizing methods returning
Self is no longer necessary, because the devirtualizer can insert the
necessary casts. In any case, the check was incorrect because we now
allow calling methods on existentials that return Self as part of
another type in covariant position, such as Optional<Self> or
`() -> Self`.
