[SE-0112] Implement ErrorProtocol.localizedDescription.

Provides a localized description for an error, using Cocoa's
error-handling logic. The first part of item (4) in SE-0112's proposed
solution.

Author: DougGregor

stdlib/public/SDK/Foundation/NSError.swift

@@ -135,6 +135,13 @@ public extension ErrorProtocol where Self : CustomNSError {
   var _code: Int { return self.errorCode }
 }
 
+public extension ErrorProtocol {
+  /// Retrieve the localized description for this error.
+  var localizedDescription: String {
+    return (self as! NSError).localizedDescription
+  }
+}
+
 /// Retrieve the default userInfo dictionary for a given error.
 @_silgen_name("swift_Foundation_getErrorDefaultUserInfo")
 public func _swift_Foundation_getErrorDefaultUserInfo(_ error: ErrorProtocol)

test/1_stdlib/ErrorProtocolBridging.swift

@@ -417,6 +417,8 @@ ErrorProtocolBridgingTests.test("Customizing NSError via protocols") {
     nsError.userInfo[NSLocalizedRecoverySuggestionErrorKey] as? String)
   expectOptionalEqual("there is no help when writing tests",
     nsError.userInfo[NSHelpAnchorErrorKey] as? String)
+  expectEqual(nsError.localizedDescription, "something went horribly wrong")
+  expectEqual(error.localizedDescription, "something went horribly wrong")
 
   // RecoverableError
   expectOptionalEqual(["Delete 'throw'", "Disable the test" ],