Merge pull request #12651 from rudkx/remove-illegal-use-of-iuo

Rework an initialization to remove a use of an IUO that was banned by…

Author: rudkx

stdlib/public/Platform/Darwin.swift.gyb

@@ -12,8 +12,7 @@
 
 @_exported import Darwin // Clang module
 
-public let MAP_FAILED =
-  UnsafeMutableRawPointer(bitPattern: -1)! as UnsafeMutableRawPointer!
+public let MAP_FAILED: UnsafeMutableRawPointer! = UnsafeMutableRawPointer(bitPattern: -1)
 
 //  Constants defined by <math.h>
 @available(swift, deprecated: 3.0, message: "Please use 'Double.pi' or '.pi' to get the value of correct type and avoid casting.")