[ParseableInterfaces] Find inherited default arg for initializer params

[harlanhaskins]

Since we try not to synthesize inherited initializers, we want to be
explicit and print them in subclasses. Since the default argument kind
was Inherited, though, they would print as

override public init(value: Swift.Int = super)

...which doesn't parse.

Instead, try to walk up the override chain for these methods and pull
out the default argument string.

Fixes rdar://49789274

[slavapestov]

This is not correct, you really need a special syntax to round-trip the inherited default arg here because of generic substitutions.

For example try this:

public protocol Initable { init() }

public class Base<T : Initable> {
  public init(t: T = T()) {}
}

extension Array : Initable {}

public class Derived : Base<[Int]> {}

You're going to print the default argument as "T()", but there's no declaration named "T" that is visible from the derived class.

[jrose-apple]

Yeah, we know. :-( But we don't have an alternative that doesn't require inventing new syntax for module interfaces. If you think this is a blocker, though, I guess we can do that.

[harlanhaskins]

Thanks for catching that, @slavapestov! We still want to avoid inventing new syntax for things only in module interfaces, so I think the only way forward is to just omit these and leave them implicit in interfaces 😓

[slavapestov]

what about an attribute on the ParamDecl?

init(@_inheritedDefaultValue t)