CGFloat: add CGFloat.init(CGFloat), which makes CGFloat similar to Float and Double

This is an API bugfix, this API was accidentally omitted when we added
CGFloat to the overlay, so no evolution proposal is necessary.

Author: gribozavr

stdlib/public/SDK/CoreGraphics/CGFloat.swift.gyb

@@ -46,6 +46,11 @@ public struct CGFloat {
     self.native = NativeType(value)
   }
 
+  @_transparent
+  public init(_ value: CGFloat) {
+    self = value
+  }
+
   /// The native value.
   public var native: NativeType
 }

test/Interpreter/SDK/CoreGraphics_CGFloat.swift

@@ -21,6 +21,7 @@ CGFloatTestSuite.test("init") {
   expectEqual(0.0, CGFloat())
   expectEqual(4.125, CGFloat(Float(4.125)))
   expectEqual(4.125, CGFloat(Double(4.125)))
+  expectEqual(4.125, CGFloat(CGFloat(Double(4.125))))
 
   expectEqual(42, CGFloat(Int(42)))
   expectEqual(42, CGFloat(Int8(42)))