Add CGFloat (and other types that this applies to) as a trivial type even after
the one-time initialization of the ForeignRepresentableCache.
This allows
let str = ""
import Foundation
let pt = CGPoint(x: 1.0, y: 2.0)
to work.
Before we would populate the cache the first time on the first line "let str =
..." and because the CoreGraphics module was not loaded we would not add CGFloat
as a trivial type. When we come to query for CGFloat on the third line we would
return NSNumber instead of CGFloat as a type and that would crash IRGen.
rdar://31610342
