Add test for construction of struct with private field with default value

```
error[E0451]: field `beta` of struct `Alpha` is private
  --> $DIR/visibility.rs:11:37
   |
LL |         let x = crate::foo::Alpha { .. };
   |                                     ^^ field `beta` is private
```

Author: estebank

tests/ui/structs/default-field-values/visibility.rs

@@ -0,0 +1,29 @@
+#![feature(default_field_values)]
+pub mod foo {
+    pub struct Alpha {
+        beta: u8 = 42,
+        gamma: bool = true,
+    }
+}
+
+mod bar {
+    fn baz() {
+        let x = crate::foo::Alpha { .. };
+        //~^ ERROR field `beta` of struct `Alpha` is private
+        //~| ERROR field `gamma` of struct `Alpha` is private
+    }
+}
+
+pub mod baz {
+    pub struct S {
+        x: i32 = 1,
+    }
+}
+fn main() {
+    let a = baz::S {
+        .. //~ ERROR field `x` of struct `S` is private
+    };
+    let b = baz::S {
+        x: 0, //~ ERROR field `x` of struct `S` is private
+    };
+}

tests/ui/structs/default-field-values/visibility.stderr

@@ -0,0 +1,27 @@
+error[E0451]: field `x` of struct `S` is private
+  --> $DIR/visibility.rs:24:9
+   |
+LL |         ..
+   |         ^^ field `x` is private
+
+error[E0451]: field `x` of struct `S` is private
+  --> $DIR/visibility.rs:27:9
+   |
+LL |         x: 0,
+   |         ^^^^ private field
+
+error[E0451]: field `beta` of struct `Alpha` is private
+  --> $DIR/visibility.rs:11:37
+   |
+LL |         let x = crate::foo::Alpha { .. };
+   |                                     ^^ field `beta` is private
+
+error[E0451]: field `gamma` of struct `Alpha` is private
+  --> $DIR/visibility.rs:11:37
+   |
+LL |         let x = crate::foo::Alpha { .. };
+   |                                     ^^ field `gamma` is private
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0451`.