stage2: LLVM backend: implement wrap_optional AIR

and move over some passing tests

test/behavior/optional.zig

@@ -2,3 +2,37 @@ const std = @import("std");
 const testing = std.testing;
 const expect = testing.expect;
 const expectEqual = testing.expectEqual;
+
+test "passing an optional integer as a parameter" {
+    const S = struct {
+        fn entry() bool {
+            var x: i32 = 1234;
+            return foo(x);
+        }
+
+        fn foo(x: ?i32) bool {
+            return x.? == 1234;
+        }
+    };
+    try expect(S.entry());
+    comptime try expect(S.entry());
+}
+
+test "self-referential struct through a slice of optional" {
+    const S = struct {
+        const Node = struct {
+            children: []?Node,
+            data: ?u8,
+
+            fn new() Node {
+                return Node{
+                    .children = undefined,
+                    .data = null,
+                };
+            }
+        };
+    };
+
+    var n = S.Node.new();
+    try expect(n.data == null);
+}