Improve parser recovery around nullable types in patterns

[DoctorKrolic]

Closes: https://github.com/dotnet/roslyn/issues/72720

[CyrusNajmabadi]

Done with pass. Overall approve of the high level idea. Def wary of the actual code change (i don't understand it yet), and def think we need to beef up tests to ensure that things that are now legal to parse give proper diagnostics.

[DoctorKrolic]

@CyrusNajmabadi PTAL

[CyrusNajmabadi]

Done with pass.

[DoctorKrolic]

@CyrusNajmabadi PTAL

[CyrusNajmabadi]

canyou ping me in a couple of days? Thnaks.

[DoctorKrolic]

@CyrusNajmabadi A few days has passed, would like your review here :)

[DoctorKrolic]

@CyrusNajmabadi I am still patiently waiting for your review here 🙂

[CyrusNajmabadi]

Ok. Pushed. I'm also ok changing the await-handling and updating the tests accordingly. Are you ok with me doing that, or would you prefer to do that?

[DoctorKrolic]

Are you ok with me doing that, or would you prefer to do that?

Go ahead. Just add a simple comment to changed tests, so it is obvious, that it is an intentional "degradation"

[CyrusNajmabadi]

Will do.

[CyrusNajmabadi]

lgtm. @dotnet/roslyn-compiler for reviews please.

[DoctorKrolic]

@333fred Can I have a review here?

[DoctorKrolic]

@333fred I noticed that there was a regression of parsing an async simple lambda in a conditional expression after pattern. Need another look from you yo approve the change

[DoctorKrolic]

@RikkiGibson You self-assigned here, can you please take a look then?

[RikkiGibson]

Thanks @DoctorKrolic!