[CodeCompletion] Be lenient in callee analysis

[rintaro]

Accept getInterfaceType()->hasError() declarations. Even if the part of the declaration has error, we still have chance to get context info from the other part of it. For instance:

  func foo(x: Int, y: INt) { }
  foo(x: #^COMPLETE^#

We should resolve Int as the context type regardless of whether parameter y is an error type or not.

[rintaro]

@swift-ci Please smoke test

[benlangmuir]

I'm okay with this, but it would be nice to not show <<error type>> anywhere (this is already an issue elsewhere in code-completion). How hard would it be to recover the original spelling and use that instead?

[rintaro]

I guess it's possible if we use TypeLoc::getTypeRepr() for error types. But that will not be a trivial change. Let me do that in another PR.

[benlangmuir]

SGTM