[PropertyWrappers] Fix a bug with class property wrapper access control

[theblixguy]

The following code currently triggers an error saying that the access level of the initializer is less than the type:

@propertyWrapper
open class OpenPropertyWrapperWithPublicInit {
  // error: public initializer 'init(wrappedValue:)' cannot have 
  //        more restrictive access than its enclosing property 
  //        wrapper type 'OpenPropertyWrapperWithPublicInit' (which is open)  
  public init(wrappedValue: String) {
    self.wrappedValue = wrappedValue
  }
  
  open var wrappedValue: String = "Hello, world"
}

If you change it from public to open, then you get an error because initializers cannot be marked open. So, you're stuck in a loop.

When looking for suitable wrapper inits, make sure we bound the check to AccessLevel::Public.

Resolves SR-11994.

[DougGregor]

We have threw same kind of getFormalAccess comparison in several places within the property wrappers code. Can you pull this fixed check into a separate static function and use it consistently throughout TypeCheckPropertyWrapper.cpp?

[theblixguy]

@DougGregor Sure, I have done that now.

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test linux

[theblixguy]

@DougGregor Shall I create a PR for 5.2 as well?

[DougGregor]

Looks great! Please do cherry-pick for 5.2