[PropertyWrappers] Look for constructors with default args when finding default init

[theblixguy]

At the moment, findDefaultInit() only looks for init() and ignores any constructors that have default args. This is a bug and it prevents the following code from working:

@propertyWrapper
struct Foo {
  let name: String

  init(name: String = "hello world") {
    self.name = name
  }

  var wrappedValue: Int {
    get { return 0 }
  }
}

struct Bar {
  @Foo var property: Int // error: property not initialized
  init() {}
}

When finding default initializers, also search for constructors with default args.

Resolves SR-11477.

[theblixguy]

I also noticed we didn't have any tests for multiple default inits, so I have now added some typechecker tests as well. I am going to run a quick test now while I wait for @DougGregor's review. Funny enough, the only way to trigger that diagnostic was to have two init() which would've triggered a redeclaration diagnostic anyway 😄

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please test source compatibility

[theblixguy]

@DougGregor Do you have any further feedback on this?

[DougGregor]

Sorry, I caused a conflict when I merged #27322

[theblixguy]

@DougGregor No problem, I have rebased now 👍 Can you take another look at this?

[theblixguy]

@DougGregor I have combined both methods into one as per your suggestion. Does it look good to you?

[DougGregor]

This is greatly improved! A few minor comments about the control flow in the unified lookup, but this will be ready to go in. Thank you!

[theblixguy]

@DougGregor Thanks for the review! I have addressed your comments :)

[theblixguy]

ping @DougGregor, can you approve this if you're happy? thank you!

[theblixguy]

A conflict creeped in - fixed it! @DougGregor just waiting for your approval :-)

[DougGregor]

Thank you! Looks fantastic.

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please test source compatibility

[theblixguy]

Debug source compat failure is an unrelated UPASS.