If there are no explicit concurrency attributes, isolated parameters,
or captures associated with the closure it should infer `nonisolated(nonsending)`
for the parent conversion injected by the solver (this conversion is injected
because the solver cannot check captures to elide it).
The change pushes `isIsolationInferenceBoundaryClosure` check down
with added benefit of getting preconcurrency context from the parent.
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
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.
