[overlay] Declaring MAP_FAILED (#4971)

Clang importer is unable to handle `((void *)-1)`, therefore manually
declaring the said constant in both Darwin and Glibc modules.

rdar://problem/26689135

Author: moiseev

stdlib/public/Platform/Darwin.swift

@@ -11,3 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 @_exported import Darwin // Clang module
+
+public let MAP_FAILED =
+  UnsafeMutableRawPointer(bitPattern: -1)! as UnsafeMutableRawPointer!

stdlib/public/Platform/Glibc.swift

@@ -11,3 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 @_exported import SwiftGlibc // Clang module
+
+public let MAP_FAILED =
+  UnsafeMutableRawPointer(bitPattern: -1)! as UnsafeMutableRawPointer!

test/stdlib/mmap.swift

@@ -0,0 +1,17 @@
+// RUN: %target-run-simple-swift %t
+// REQUIRES: executable_test
+
+import StdlibUnittest
+#if os(Linux)
+  import Glibc
+#else
+  import Darwin
+#endif
+
+var MMapTests = TestSuite("MMaptests")
+
+MMapTests.test("MAP_FAILED") {
+  expectEqual(mmap(nil, 0, 0, 0, 0, 0), MAP_FAILED)
+}
+
+runAllTests()