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Issue Description:

When Obj-C declares a completion block that takes e.g. an NSDictionary, and Swift code provides the closure, the compiler automatically bridges the parameters from Obj-C to Swift, turning that NSDictionary into [AnyHashable: Any] (or in Swift 2.3, [NSObject: AnyObject]). In Swift 2.3, if the block ignores the parameter, this bridging does not actually happen. In Swift 3.0, it happens even though the parameter is unused. Not only is this unnecessary work that can adversely affect performance, it's also a potential crash if the Obj-C code passes an object of the wrong type to the block (which is what actually happened in our app).

I've attached an Xcode project demonstrating this. It declares an Obj-C class that accepts a completion block, and it passes an object of the wrong type to the completion block. It has schemes for both Swift 2.3 and Swift 3.0. In Swift 2.3, the program executes just fine (since it ignores the parameter). In Swift 3.0 it crashes in Dictionary._unconditionallyBridgeFromObjectiveC because it's sending an unrecognized selector to NSData. The Swift code looks like

BadClass().crash({ _ in
    print("this block ignores its parameter")
})