[codecomplete] Fix unresolved member completion for T within Optional<T>

[benlangmuir]

We need to look through the optional and find the members of T when
doing completion in Optional.

let x: Foo? = .foo

We still don't correctly complete .some/.none, which requires
reconciling the unbound generic type we get from the decl with the real
bound generic type.

rdar://44767478

[benlangmuir]

@swift-ci please smoke test

[rintaro]

Makes sense!

[rintaro]

I will see what we can do for .some/.none.
It also affects other completions, like:

struct Foo<T> {
  static var instance: Foo<T> { return Foo<T>() }
}

let _: Foo<Int> = .#^COMPLETE^# // Currently, this doesn't work.