Fix invalid returns in class getter's lazy evaluation blocks

[ysbaddaden]

Using return in a class getter will immediately return from the generated method, never setting the class variable to the returned value, which is kept nil so the initializer code is called repeatedly instead of being called once then cached.

Extracted from #15340

[RX14]

I suspect this could be quite a common breaking change in practice, I'd rather not make this change if at all possible. It also adds another footgun for future implementations that isn't easily caught. Or does this still happen with the current class getter impl?

[ysbaddaden]

@RX14 This patch only fixes a couple identified issues, whatever if we start capturing the block or not: the behavior was wrong.

[straight-shoota]

Yeah this is not changing anything, it just fixes incorrect uses of return in a class getter block. They are already broken: return exits from the outer context and thus doesn't assign the ivar, which means the lazy initialization evaluates over and over again.
So you basically never want that.