Support if statements in dataclass_transform class

[KRunchPL]

Fixes: #14853

Adding support for if statements in the dataclass_transform class, so the attributes defined conditionally are treated as those that are directly in class body.

[sobolevn]

Thanks! The usecase with if TYPE_CHECKING seems reasonable to me.
I personally use this from time to time.

[sobolevn]

Let's add at least a single test with some def cond() -> bool: ... function.

if cond():
   x: int
   y: int
   z1: int
else:
   x: str
   y: int
   z2: int

[KRunchPL]

I have added the test. It is of course expecting errors, because mypy cannot know what is the return value of a function right? The condition logic is based on IfStmt.is_unreachable which is filled in by logic found in mypy.reachability. I have added a test with some comments that not only tests, but also shows the behavior. I am not sure if we can avoid this limitation (although I am new here, so I can be mistaken). And of course most typical use case will be if TYPE_CHECKING, which we can know is ALWAYS_TRUE.

[sobolevn]

Yes, I think the test is correct 👍

[sobolevn]

One last thing from me: as far as I understand - this does also affect how @dataclasses are defined.

Can we please add a single test for it as well?

Thanks a lot for your work! 👍

[KRunchPL]

Added the test for pute dataclass.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[sobolevn]

Thank you!