[ty] Handle annotated self parameter in constructor of non-invariant generic classes (#21325)

This manifested as an error when inferring the type of a PEP-695 generic
class via its constructor parameters:

```py
class D[T, U]:
    @overload
    def __init__(self: "D[str, U]", u: U) -> None: ...
    @overload
    def __init__(self, t: T, u: U) -> None: ...
    def __init__(self, *args) -> None: ...

# revealed: D[Unknown, str]
# SHOULD BE: D[str, str]
reveal_type(D("string"))
```

This manifested because `D` is inferred to be bivariant in both `T` and
`U`. We weren't seeing this in the equivalent example for legacy
typevars, since those default to invariant. (This issue also showed up
for _covariant_ typevars, so this issue was not limited to bivariance.)

The underlying cause was because of a heuristic that we have in our
current constraint solver, which attempts to handle situations like
this:

```py
def f[T](t: T | None): ...
f(None)
```

Here, the `None` argument matches the non-typevar union element, so this
argument should not add any constraints on what `T` can specialize to.
Our previous heuristic would check for this by seeing if the argument
type is a subtype of the parameter annotation as a whole — even if it
isn't a union! That would cause us to erroneously ignore the `self`
parameter in our constructor call, since bivariant classes are
equivalent to each other, regardless of their specializations.

The quick fix is to move this heuristic "down a level", so that we only
apply it when the parameter annotation is a union. This heuristic should
go away completely 🤞 with the new constraint solver.

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -674,8 +674,7 @@ x6: Covariant[Any] = covariant(1)
 x7: Contravariant[Any] = contravariant(1)
 x8: Invariant[Any] = invariant(1)
 
-# TODO: This could reveal `Bivariant[Any]`.
-reveal_type(x5)  # revealed: Bivariant[Literal[1]]
+reveal_type(x5)  # revealed: Bivariant[Any]
 reveal_type(x6)  # revealed: Covariant[Any]
 reveal_type(x7)  # revealed: Contravariant[Any]
 reveal_type(x8)  # revealed: Invariant[Any]