[Concurrency] Handle cases where a property initializer is subsumed by another property for IsolatedDefaultValues.

[hborla]

When determining whether compiler synthesized initializers should have nonisolated applied, skip property initializers that are subsumed, e.g. by an init accessor or a backing property wrapper initializer, and always consider the subsuming initializer. This fixes an issue where the compiler would apply nonisolated to memberwise and default initializers of types that use property wrappers whose backing property wrapper initializer is global actor isolated, but the wrapped value initializer is not, e.g.

@MainActor
@propertyWrapper struct RequiresMain<Value>  {
  var wrappedValue: Value

  init(wrappedValue: Value) {
    self.wrappedValue = wrappedValue
  }
}

// `S.init()` should be isolated to `@MainActor` because 
// `RequiresMain.init(wrappedValue:)` is isolated to `@MainActor`
struct S {
  @RequiresMain private var x = 10
}

This change also lessens the source break of SE-0411 by still emitting member initializers in implicit constructors when the initializer violates actor isolation to preserve the behavior of existing code when concurrency diagnostics are downgraded to warnings in Swift 5 mode.

Resolves rdar://121282537

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please test source compatibility

[hborla]

Those source compatibility failures are real. This fails under -strict-concurrency=complete

struct CError: Error, RawRepresentable {
  var rawValue: CInt
}

Probably because I'm calling getAllMembers() which invokes the conformance synthesis earlier. Instead, I can iterate over the stored properties and the init accessor properties, because those are what can participate in the implicit initializers.

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please test source compatibility