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

[hborla]

• Explanation: SE-0411 applies nonisolated to implicit initializers when the initializer does not require actor isolation because all property types are Sendable with nonisolated initializer expressions. This was done by looking at the properties and initial values from the memberwise initializer. However, property wrappers and init accessors can have an actor isolation that's different from the initializer expression that it subsumes. If the backing property wrapper initializer is global actor isolation but the wrapped value initializer is not, the compiler incorrectly applied nonisolated to implicit initializers, and SILGen later errored because the property wrapper backing initializer was skipped due to mismatching isolation, e.g.

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

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

struct S { // error: return from initializer without initializing all stored properties
  @RequiresMain private var x = 10
}

To fix this, skip property initializers that are subsumed, e.g. by an init accessor or a backing property wrapper initializer, and always consider the subsuming initializer to determine whether an implicit initializer can be nonisolated.

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.

• Scope: Only impacts code built with -strict-concurrency=complete or -enable-upcoming-feature IsolatedDefaultValues that uses global-actor-isolated property wrappers with Sendable, nonisolated wrapped value expressions and implicit initializers.
• Risk: Low risk; this change is relatively narrow in scope, and I've also downgraded the relevant compiler errors to warnings until Swift 6 to prevent any other implicit initializer bugs from causing source breakage. Note that the source break mentioned in [5.10][SE-0411] Promote IsolatedDefaultValues from an experimental feature to an upcoming feature. #70839 still applies for explicit initializers; the bug here was only with actor isolation inference for implicit initializers.
• Testing: Added new tests. Passed source compatibility testing on the main PR.
• Reviewer: @DougGregor
• Main branch PR: [Concurrency] Handle cases where a property initializer is subsumed by another property for IsolatedDefaultValues. #71033

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test