[Diagnostics] Fix missing any warnings for type aliases.

[hborla]

Only warn when using a type alias if the underlying type is directly a constraint type, e.g.

protocol P {
  associatedtype A
}

typealias Constraint = P

let x: Constraint // warning for missing 'any' on 'Constraint'

Also check the underlying type at the declaration of a type alias for missing 'any' if the underlying type is not directly a constraint type, e.g.

protocol P {
  associatedtype A
}

typealias Function = (P) -> Void // warning for missing 'any' on 'P'

Resolves: rdar://88980470

[hborla]

@swift-ci please smoke test