Add new source file Macros.swift

Author: DougGregor

stdlib/public/core/Macros.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// Specifies the module and type name for an externally-defined macro, which
+/// must conform to the appropriate set of `Macro` protocols.
+///
+/// This macro can only be used to define other macros. For example:
+///
+///    macro stringify<T>(_ value: T) -> (T, String) =
+///        #externalMacro(module: "ExampleMacros", type :"StringifyMacro")
+///
+/// Use of this macro in any other context is an error.
+@expression
+public macro externalMacro<T>(module: String, type: String) -> T =
+    Builtin.ExternalMacro
+