Check coroutine upvars in dtorck constraint

[compiler-errors]

Fix #144155.

This PR fixes an unsoundness where we were not considering coroutine upvars as drop-live if the coroutine interior types (witness types) had nothing which required drop.

In the case that the coroutine does not have any interior types that need to be dropped, then we don't need to treat all of the upvars as use-live; instead, this PR uses the same logic as closures, and descends into the upvar types to collect anything that must be drop-live. The rest of this PR is reworking the comment to explain the behavior here.

r? @lcnr or reassign 😸

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Just some thoughts --- a proper fix for this whole situation would be to consider TypingMode in the needs_drop function, and just calling coroutine_ty.needs_drop(tcx, typing_env) in the dtorck constraint check.

During MIR building, we should probably use a typing mode that stalls the local coroutines and considers them to be unconditionally drop, or perhaps just stall all coroutines in analysis mode. Then in borrowck mode, we can re-check needs_drop but descend into witness types properly. #144158 implements this experimentally.

This is a pretty involved fix, and conflicts with some in-flight changes (#144157) that I have around removing coroutine witnesses altogether. I'm happy to add a FIXME to rework this whole approach, but I don't want to block this quick fix since it's obviously more correct than the status-quo.