Special-case value-expression inference of special form subscriptions (#16877)

## Summary

Currently for something like `X = typing.Tuple[str, str]`, we infer the
value of `X` as `object`. That's because `Tuple` (like many of the
symbols in the typing module) is annotated as a `_SpecialForm` instance
in typeshed's stubs:


https://github.com/astral-sh/ruff/blob/23382f5f8c7b4e356368cdeb1049b8c1910baff3/crates/red_knot_vendored/vendor/typeshed/stdlib/typing.pyi#L215

and we don't understand implicit type aliases yet, and the stub for
`_SpecialForm.__getitem__` says it always returns `object`:


https://github.com/astral-sh/ruff/blob/23382f5f8c7b4e356368cdeb1049b8c1910baff3/crates/red_knot_vendored/vendor/typeshed/stdlib/typing.pyi#L198-L200

We have existing false positives in our test suite due to this:


https://github.com/astral-sh/ruff/blob/23382f5f8c7b4e356368cdeb1049b8c1910baff3/crates/red_knot_python_semantic/resources/mdtest/annotations/annotated.md?plain=1#L76-L78

and it's causing _many_ new false positives in #16872, which tries to
make our annotation-expression parsing stricter in some ways.

This PR therefore adds some small special casing for `KnownInstanceType`
variants that fallback to `_SpecialForm`, so that these false positives
can be avoided.

## Test Plan

Existing mdtest altered.

Cc. @MatthewMckee4

Author: AlexWaygood

crates/red_knot_python_semantic/resources/mdtest/annotations/annotated.md

@@ -73,12 +73,10 @@ Inheriting from `Annotated[T, ...]` is equivalent to inheriting from `T` itself.
 ```py
 from typing_extensions import Annotated
 
-# TODO: False positive
-# error: [invalid-base]
 class C(Annotated[int, "foo"]): ...
 
 # TODO: Should be `tuple[Literal[C], Literal[int], Literal[object]]`
-reveal_type(C.__mro__)  # revealed: tuple[Literal[C], Unknown, Literal[object]]
+reveal_type(C.__mro__)  # revealed: tuple[Literal[C], @Todo(Inference of subscript on special form), Literal[object]]
 ```
 
 ### Not parameterized

crates/red_knot_python_semantic/src/types/class.rs

@@ -851,6 +851,10 @@ impl<'db> KnownClass {
         matches!(self, Self::Bool)
     }
 
+    pub(crate) const fn is_special_form(self) -> bool {
+        matches!(self, Self::SpecialForm)
+    }
+
     /// Determine whether instances of this class are always truthy, always falsy,
     /// or have an ambiguous truthiness.
     pub(crate) const fn bool(self) -> Truthiness {

crates/red_knot_python_semantic/src/types/infer.rs

@@ -5631,6 +5631,11 @@ impl<'db> TypeInferenceBuilder<'db> {
             (Type::KnownInstance(KnownInstanceType::Protocol), _) => {
                 Type::Dynamic(DynamicType::TodoProtocol)
             }
+            (Type::KnownInstance(known_instance), _)
+                if known_instance.class().is_special_form() =>
+            {
+                todo_type!("Inference of subscript on special form")
+            }
             (value_ty, slice_ty) => {
                 // If the class defines `__getitem__`, return its return type.
                 //