swift234

Fix <rdar://problem/23202128> QoI: Poor diagnostic with let vs. var passed to C function

This improves the error message when attempting an array to UnsafeMutablePointer
conversion but where the element type of the array is incorrect or where the array itself
is immutable.

As a bonus fix, this dramatically improves the diagnostic when you pass "&array" to
a function that takes an UnsafePointer.  We decided to not require & in this case, so
we can just provide a nice fixit to rip it off when this common error happens.