The reason why I am doing this is that in certain cases the AST captures indices
will never actually line up with partial apply capture indices since we seem to
"smush" together closures and locally defined functions.
NOTE: The reason for the really small amount of test changes is that this change
does not change the actual output by design. The only cases I had to change were
a case where we began to emit a better diagnostic and also where I added code
coverage around _ and let _ since those require ignored_use to be implemented so
that they would be diagnosed (previously we just did not emit anything so we
couldn't emit the diagnostic at the SIL level).
rdar://142661388
Instead, use the `%target-swift-5.1-abi-triple` substitution to compile the tests
for deployment to the minimum OS versions required for use of _Concurrency APIs.
The reason why I am fixing this is that otherwise, we get a warning when one
creates an actor isolated closure and pass it into a task, e.x.:
```swift
@MainActor func test() {
// We would get a warning on the closure below saying that we are sending
// a closure that is MainActor isolated.
Task {
...
}
}
```
We view the conversion from a Sendable to a non-Sendable function via
convert function to produce a new fresh sendable value. We should
squelch that error.
Consider the following piece of code and what the isolation is of the closure
literal passed to doSomething():
```swift
func doSomething(_ f: sending () -> ()) { ... }
@MyCustomActor
func foo() async {
doSomething {
// What is the isolation here?
}
}
```
In this case, the isolation of the closure is @MyCustomActor. This is because
non-Sendable closures are by default isolated to their current context (in this
case @MyCustomActor since foo is @MyCustomActor isolated). This is a problem
since
1. Our closure is a synchronous function that does not have the ability to hop
to MyCustomActor to run said code. This could result in a concurrency hole
caused by running the closure in doSomething() without hopping to
MyCustomActor's executor.
2. In Region Based Isolation, a closure that is actor isolated cannot be sent,
so we would immediately hit a region isolation error.
To fix this issue, by default, if a closure literal is passed as a sending
parameter, we make its isolation nonisolated. This ensures that it is
disconnected and can be transferred safely.
In the case of an async closure literal, we follow the same semantics, but we
add an additional wrinkle: we keep support of inheritActorIsolation. If one
marks an async closure literal with inheritActorIsolation, we allow for it to be
passed as a sendable parameter since it is actually Sendable under the hood.
