Optional Class (#2096)

Adding Optional class to prelude...
I hope it is useful!

Co-authored-by: m new <michael.burzan@outlook.de>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Geoff Romer <gromer@google.com>

explorer/testdata/optional/init_optional_with_non_value.carbon

@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32{
+  var x: Optional(String) = Optional(String).CreateEmpty();
+  if(x.HasValue()){
+    return -1;
+  }
+  return 0;
+}