Issue Description:

Hi Apple,

Something that comes up frequently when using pointers in Swift is the you can not return an UnsafeMutablePointer where an UnsafePointer is expected. In C, mutable pointers can be converted implicitly to "const" pointers. Would it be possible to add such a conversion to Swift without degrading type checking performance? I guess the same would go for UnsafeMutableRawPointers. This would help with code using imports of pointers from C which frequently have reasonably arbitrary mutability.

Cheers,

John