[SR-4949] No error for using 'as' patterns with CF types

Previous ID	SR-4949
Radar	rdar://problem/32255478
Original Reporter	@belkadan
Type	Bug

Votes	0
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, StarterBug
Assignee	None
Priority	Medium

md5: 62d9e9e2da2d4b28fa3d10ebd350dba6

Issue Description:

import Foundation

let x = ([1: 2] as NSDictionary) as Any

switch x {
case let b as CFBoolean:
    print("it's a cfbool: \(b)")
    
default:
    print("n/a")
}

We should warn here that we can't actually do checked downcasts for CF types, like we do with as?.

import Foundation
let x = ([1: 2] as NSDictionary) as Any
let y = x as? CFDictionary // error: conditional downcast to CoreFoundation type 'CFDictionary' will always succeed

We can't make it an error because that would break source compatibility, but at least a warning will work. The best workaround today (and a possible fix-it):

switch x {
case let rawB:
    let b = rawB as! CFBoolean
    print("it's a cfbool: \(b)")
    
default:
    print("n/a")
}