Collection expressions: handle use site diagnostics and other complex cases when using well-known members

[RikkiGibson]

Regarding issues with use-site diagnostics, Obsolete, constraint checks, etc. I think we should punt them to 17.9. I'm happy to take on that work. I think it is OK for the the tooling to behave badly in the short term if someone implements this framework-level API in an unexpected fashion. e.g. producing an assembly that fails at runtime, or missing expected diagnostics at compile time.

Originally posted by @RikkiGibson in #70222 (comment)

Relates to test plan: #66418

[jcouv]

Part of this issue relates to Obsolete and constraints checks on well-known types/members. We discussed this topic earlier today with Jared/Aleksey/Fred and I wanted to record the team's stance: we're okay not to check Obsolete, UnmanagedCallersOnly or unexpected constraints on well-known types/members. I'm in process of documenting that.

[RikkiGibson]

What remains in this issue that we would actually want to implement? Checking use site diagnostics?

[AlekseyTs]

And collecting dependencies