Add codegen test

Author: Aaron1011

src/test/codegen/fn-impl-trait-self.rs

@@ -0,0 +1,15 @@
+// compile-flags: -g
+//
+// CHECK-LABEL: @main
+// CHECK: {{.*}}DIDerivedType(tag: DW_TAG_pointer_type, name: "fn() -> <recursive_type>",{{.*}}
+//
+// CHECK: {{.*}}DISubroutineType{{.*}}
+// CHECK: {{.*}}DIBasicType(name: "<recur_type>", encoding: DW_ATE_unsigned)
+
+pub fn foo() -> impl Copy {
+    foo
+}
+
+fn main() {
+    let my_res = foo();
+}