[CodeCompletion] Avoid prioritizing unavailable in LookupVisibleDecls

[rintaro]

public protocol P {}
public extension P {
  @available(*, unavailable)
  func foo() {}
}
struct S: P {
  func foo() {}
}
func test(value: S) {
  value.<HERE>
}

foo() wasn't suggested because lookupVisibleMemberDecls() returned extension P.foo() which has wider accessibility public. But code completion doesn't emit unavailable candidates.

rdar://problem/67155695

[rintaro]

@swift-ci Please smoke test