[ExprContextAnalysis] Look into base type of optional as well when collecting callees for unresolved member

[theblixguy]

When collecting callees for an unresolved member (like .foo), if the type is optional then also check the base type for any callees. Otherwise, code completion will not be able to suggest argument labels for a call.

struct S {
  static func foo(arg: Int) -> Self { return S() }
}

let s: S? = .foo(#^COMPLETE^#) // should suggest 'arg'

Resolves SR-13340
Resolves rdar://problem/66490100

[rintaro]

Looks great! Thank you!

[theblixguy]

@swift-ci please smoke test

Windows: https://ci-external.swift.org/view/Pull%20Request/job/swift-PR-windows/5971/