Add parameter name hints for discriminated unions

[kerams]

Implements #14257.

[kerams]

Should be good to go unless there's some other way of using DUs that I didn't think of.

[psfinaki]

Kudos for jumping on that, @kerams!

Good testing and optimizations, and also thanks for keeping the original errh "design" - we'll see how long it's going to make sense :)

[psfinaki]

Should be good to go unless there's some other way of using DUs that I didn't think of.

@kerams so actually, just before I wanted to reapprove the PR, I opened our own repo with these DU hints on and discovered that there are some places that cause crashes now. The easiest repro is the following:

type X =
    member _.Test() = 42 :: [42; 42]

So yeah we should handle and test this :)

[kerams]

Hehe, to be fair it's GetAllArgumentsForFunctionApplicationAtPosition that's failing, not my code :P. Good catch though.

I think Cons needs to be special-cased here and ignored.

[psfinaki]

Thanks again - btw while testing parameter name hints I actually found a bug in type hints, should be also an easy fix.