Use actual inference to determine dataclasses

[DanielNoord]

Steps to reproduce

def test_non_dataclass_is_not_dataclass() -> None:
    """Test that something that isn't a dataclass has the correct attribute."""
    code = astroid.extract_node(
        """
    @dataclass
    class A:
        val: field()
    """
    )
    inferred = code.inferred()
    assert len(inferred) == 1
    assert isinstance(inferred[0], nodes.ClassDef)
    assert not inferred[0].is_dataclass

With the changes in #1391 this should pass, but it doesn't.
I think it might be because we only check the name of the decorator and not actually what it is. ie., in this case dataclass is not imported so A won't be a dataclass.

Expected behavior

Determine what is and what isn't a dataclass a little better.

[cdce8p]

Actually, the decorator nodes are already inferred. In this instance it just triggers the fallback since @dataclass is Uninferable.
https://github.com/PyCQA/astroid/blob/552f1c19aab0719e7b30bb02e032d4bfe655f343/astroid/brain/brain_dataclasses.py#L285-L292

Just add a function named dataclass and the test will pass.

    def dataclass(c):
        return c

[DanielNoord]

Ah, my bad! That does feel a bit weird thought, but there is probably a reason for why this was added!

I'll close and add a test.