[Concurrency] Emit a better note when an isolated stored property initializer cannot be used.

[hborla]

When a stored property initializer requires a specific isolation, an init with mismatching isolation cannot use that initial value. Today, the compiler emits misleading notes in that situation. For example:

@MainActor func f() -> Int { 0 }

class C {
  @MainActor var x = f()

  init() {}
}

Currently, the compiler emits the following diagnostics:

error: return from initializer without initializing all stored properties
2 | 
3 | class C {
4 |   @MainActor var x = f()
  |                  `- note: 'self.x' not initialized
5 | 
6 |   init() {}
  |           `- error: return from initializer without initializing all stored properties
7 | }
8 | 

The attached note is misleading, because x sure looks initialized at the declaration! The real problem is that the initializer is @MainActor-isolated, so it cannot be used in a nonisolated initializer. With this change, the attached note includes the isolation of the default value and the enclosing init that has mismatching isolation:

error: return from initializer without initializing all stored properties
2 | 
3 | class C {
4 |   @MainActor var x = f()
  |                  `- note: main actor-isolated default value of 'self.x' cannot be used in a nonisolated initalizer
5 | 
6 |   init() {}
  |           `- error: return from initializer without initializing all stored properties
7 | }
8 | 

Resolves: #74557

(Note that this PR includes the commits from #75687. Only the last commit is new)

[hborla]

@swift-ci please smoke test