Add support for return; (#678)

This creates a ReturnExpression. A separate change should enforce that an implicit return of `()` is only allowed in functions that have an implicit return type of `()`, and I think the structure taken here should ease that.

executable_semantics/ast/expression.cpp

@@ -100,12 +100,17 @@ auto Expression::MakeContinuationTypeLiteral(int line_num)
   return type;
 }
 
-auto Expression::MakeFunctionTypeLiteral(int line_num, const Expression* param,
-                                         const Expression* ret)
+auto Expression::MakeFunctionTypeLiteral(int line_num,
+                                         const Expression* parameter,
+                                         const Expression* return_type,
+                                         bool is_omitted_return_type)
     -> const Expression* {
   auto* t = global_arena->New<Expression>();
   t->line_num = line_num;
-  t->value = FunctionTypeLiteral({.parameter = param, .return_type = ret});
+  t->value =
+      FunctionTypeLiteral({.parameter = parameter,
+                           .return_type = return_type,
+                           .is_omitted_return_type = is_omitted_return_type});
   return t;
 }