only warn for isolation mismatch in @preconcurrency context
Much like how we do for an isolation mismatch for a call within
a closure that is `@preconcurrency`, references should also only
warn about the isolation issue too.
The reason why there is a mismatch is that the completion-handlers
are considered `@Sendable`, which disables the isolation inheriting
behavior of other, non-Sendable closures.
resolves rdar://96830159
