[ty] Track when type variables are inferable or not (#19786)

`Type::TypeVar` now distinguishes whether the typevar in question is
inferable or not.

A typevar is _not inferable_ inside the body of the generic class or
function that binds it:

```py
def f[T](t: T) -> T:
    return t
```

The infered type of `t` in the function body is `TypeVar(T,
NotInferable)`. This represents how e.g. assignability checks need to be
valid for all possible specializations of the typevar. Most of the
existing assignability/etc logic only applies to non-inferable typevars.

Outside of the function body, the typevar is _inferable_:

```py
f(4)
```

Here, the parameter type of `f` is `TypeVar(T, Inferable)`. This
represents how e.g. assignability doesn't need to hold for _all_
specializations; instead, we need to find the constraints under which
this specific assignability check holds.

This is in support of starting to perform specialization inference _as
part of_ performing the assignability check at the call site.

In the [[POPL2015][]] paper, this concept is called _monomorphic_ /
_polymorphic_, but I thought _non-inferable_ / _inferable_ would be
clearer for us.

Depends on #19784 

[POPL2015]: https://doi.org/10.1145/2676726.2676991

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/src/semantic_model.rs

@@ -232,7 +232,7 @@ impl<'db> Completion<'db> {
                 | Type::BytesLiteral(_) => CompletionKind::Value,
                 Type::EnumLiteral(_) => CompletionKind::Enum,
                 Type::ProtocolInstance(_) => CompletionKind::Interface,
-                Type::TypeVar(_) => CompletionKind::TypeParameter,
+                Type::NonInferableTypeVar(_) | Type::TypeVar(_) => CompletionKind::TypeParameter,
                 Type::Union(union) => union.elements(db).iter().find_map(|&ty| imp(db, ty))?,
                 Type::Intersection(intersection) => {
                     intersection.iter_positive(db).find_map(|ty| imp(db, ty))?