[concurrency] type-checking global-actor-isolated references in ordinary functions

[kavon]

Non-actor isolated synchronous functions were mistakenly allowed to call & reference global-actor isolated declarations:

actor class TestActor {}

@globalActor
struct SomeGlobalActor {
  static var shared: TestActor { TestActor() }
}

@SomeGlobalActor func syncGlobActorFn() { }

func ordinaryFunc() {
  syncGlobActorFn() // reference A
  let x = syncGlobActorFn // reference B
  x()
}

There are two invalid references to global-actor isolated declarations. There are various ways to fix such problems:

1. marking ordinaryFunc as @asyncHandler
2. marking ordinaryFunc as async
3. making ordinaryFunc part of the global-actor that syncGlobActorFn belongs to.

We only suggest fix (3) if the incorrect global-actor reference is not part of an apply (i.e., reference B). For reference A, which directly applies the function, we suggest all 3 as possible fixes.

Thus, in terms of new fix-its: If the caller is not associated with an actor and is a synchronous function that refers to a global-actor isolated declaration, we will suggest the option of making that synchronous function part of the global-actor's context (fixing rdar://71698464). We do not suggest becoming part of the global-actor if the reference appears in an async context.

Resolves rdar://71548470&71698464

TODOs:

• enhance regression test with fix-it checking (+ manually verify that they produce valid code)

[kavon]

@swift-ci please smoke test

[kavon]

@swift-ci please smoke test

[kavon]

@swift-ci please smoke test

[kavon]

@swift-ci please smoke test and merge