Special-case value-expression inference of special form subscriptions

[AlexWaygood]

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:

ruff/crates/red_knot_vendored/vendor/typeshed/stdlib/typing.pyi

Line 215 in 23382f5

Tuple: _SpecialForm

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

ruff/crates/red_knot_vendored/vendor/typeshed/stdlib/typing.pyi

Lines 198 to 200 in 23382f5

	@final
	class _SpecialForm(_Final):
	def __getitem__(self, parameters: Any) -> object: ...

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

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

Lines 76 to 78 in 23382f5

	# TODO: False positive
	# error: [invalid-base]
	class C(Annotated[int, "foo"]): ...

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

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

black (https://github.com/psf/black)
- error[lint:non-subscriptable] /tmp/mypy_primer/projects/black/src/black/trans.py:54:11: Cannot subscript object of type `object` with no `__getitem__` method
- Found 289 diagnostics
+ Found 288 diagnostics

[AlexWaygood]

mypy_primer results

Changes were detected when running on open source projects

black (https://github.com/psf/black)
- error[lint:non-subscriptable] /tmp/mypy_primer/projects/black/src/black/trans.py:54:11: Cannot subscript object of type `object` with no `__getitem__` method
- Found 289 diagnostics
+ Found 288 diagnostics

Here's the link to the line in question: https://github.com/psf/black/blob/dd278cb316d75868716a0478c35b1fcd600a5249/src/black/trans.py#L54

Result is a type alias to a Union[] subscription, and Union is stubbed as an instance of _SpecialForm in typeshed:

https://github.com/psf/black/blob/dd278cb316d75868716a0478c35b1fcd600a5249/src/black/rusty.py#L28

So it's good that we no longer emit this error!