This matches send non sendable but importantly also makes it clear that we are
talking about something that doesn't conform to the Sendable protocol which is
capitalized.
rdar://151802975
The original check examined only the immediate closure, but it's
possible that the closure happens to be in a preconcurrency context
which also requires a downgrade.
Resolves: rdar://148996589
Sendable violations inside `@preconcurrency @Sendable` closures should be
suppressed in minimal checking, and diagnosed as warnings under complete
checking, including the Swift 6 language mode.
