[red-knot] Detect unreachable attributes assignments

[sharkdp]

This is probably not a high-priority feature, but it would be great if we could detect attribute assignments in unreachable code sections, and eliminate them from the set of possible answers:

class C:
    def __init__(self) -> None:
        if True:
            self.x = 1
        else:
            self.x = "a"

        return

        self.y = "unreachable"

reveal_type(C().x)  # should be `Unknown | Literal[1]` (i.e. not include `Literal["a"]`)

# Should be an error:
C().y

Note that we have one existing test for this here:

ruff/crates/red_knot_python_semantic/resources/mdtest/attributes.md

Lines 434 to 448 in 16f2a93

	```py
	class C:
	def __init__(self) -> None:
	# We use a "significantly complex" condition here (instead of just `False`)
	# for a proper comparison with mypy and pyright, which distinguish between
	# conditions that can be resolved from a simple pattern matching and those
	# that need proper type inference.
	if (2 + 3) < 4:
	self.x: str = "a"
	# TODO: Ideally, this would result in a `unresolved-attribute` error. But mypy and pyright
	# do not support this either (for conditions that can only be resolved to `False` in type
	# inference), so it does not seem to be particularly important.
	reveal_type(C().x) # revealed: str
	```