subtype: Keep precise track of subtype meets (#61940)
When subtyping accumulates its constraints on existential vars, it needs
to union all lower bound constraints and intersect all upper bound
constraints. Currently, this is done with simple_meet (or, if in
intersection with the full intersection algorithm). The problem with
simple_meet is that it can overapproximate the intersection, which as
shown in #61917 can lose constraints and give wrong subtyping answers.
The solution is simple: If `simple_meet` cannot compute an exact
intersection, allocate a new object that keeps a list of all the
constraints. This object is semantically an intersection type. However,
our use here is very limited: It only ever occurs in the `ub` of a
`varbinding` and thus only ever occurs on the RHS of a subtype query.
Fortunately for us, that is the simple direction: `A <: Intersect{X, Y}
<=> A <: X && A <: Y`. It's therefore mostly used as a bookkeeping
device rather than being a full extention to the type system. However,
this PR does style it in such a way that it could be extended to a full
typesystem feature in the future if we were so inclined (though I have
no present plans to do so).
I should also note that the formation of `Intersect` is pretty rare. In
the whole Compiler/Base bootstrap, it happens exactly twice out of
millions of subtype queries.
Fixes #61917. Code by Claude.
