`nonisolated(nonsending)` → `@isolated(any)` conversion uses nil actor

[AnthonyLatsis]

Description

We should be partially applying a nonisolated(nonsending) function to the actor it should run on, if any, before converting to @isolated(any). Instead, we are unconditionally applying it a nil actor.

Reproduction

func test(_ fn: @isolated(any) () async -> Void) {
  print(fn.isolation) // nil
}

nonisolated(nonsending) func foo() async {
  test {}
}

@main struct Foo {
  @MainActor
  static func main() async throws {
    await foo()
  }
}

Expected behavior

MainActor in this case.

Environment

a868d92.

Additional information

No response

[xedin]

cc @gottesmm