[6.1] Fix some type issues around handling closures in SendNonSendable #79164
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This change fixes a logic error where we were not correctly checking if a type was non-Sendable by converting a potentially not lowered type from the AST to a SILType and then checking non-Sendability with that. Since the type was not lowered in cases where the formal and lowered type of a capture differed, we crashed. Instead, we now perform the check at the CanType level with formal types.
On main, I have completely eliminated this code path by just using a SIL level check (since it also fixed another unrelated issue). This was viewed as too risky for 6.1 (#78990), so I went back in and redid it just using types.
Risk: Low
Testing: Added compiler tests.
Issue: rdar://142661388
Reviewer: @xedin
Original PRs: N/A