[CoreGraphics] Make CGColor adopt _ExpressibleByColorLiteral.

The backflips here are because we cannot add initializers to imported
CF types; they would be factory initializers. Fixes
rdar://problem/32196175.

Author: DougGregor

stdlib/public/SDK/CoreGraphics/CoreGraphics.swift

@@ -48,6 +48,19 @@ extension CGColor {
 #endif
 }
 
+public protocol _CGColorInitTrampoline {
+  init(red: CGFloat, green: CGFloat, blue: CGFloat, alpha: CGFloat)
+}
+
+extension _CGColorInitTrampoline {
+  public init(colorLiteralRed red: Float, green: Float, blue: Float,
+              alpha: Float) {
+    self.init(red: CGFloat(red), green: CGFloat(green), blue: CGFloat(blue),
+              alpha: CGFloat(alpha))
+  }
+}
+
+extension CGColor : _CGColorInitTrampoline, _ExpressibleByColorLiteral { }
 
 //===----------------------------------------------------------------------===//
 // CGColorSpace

validation-test/stdlib/CoreGraphics-execute.swift

@@ -48,6 +48,17 @@ CoreGraphicsTests.test("CGColor.components") {
   expectEqual(components[3], 1)
 }
 
+CoreGraphicsTests.test("CGColor/ExpressibleByColorLiteral") {
+  let colorLit: CGColor = #colorLiteral(red: 0.25, green: 0.5, blue: 0.75,
+                                        alpha: 1.0)
+  let components = colorLit.components!
+  expectEqual(components.count, 4)
+  expectEqual(components[0], 0.25)
+  expectEqual(components[1], 0.50)
+  expectEqual(components[2], 0.75)
+  expectEqual(components[3], 1.0)
+}
+
 //===----------------------------------------------------------------------===//
 // CGPoint
 //===----------------------------------------------------------------------===//