Add test for 'std' crate being public

Author: Aaron1011

src/test/ui/privacy/pub-priv-dep/std-pub.rs

@@ -0,0 +1,13 @@
+// The 'std' crates should always be implicitly public,
+// without having to pass any compiler arguments
+
+// run-pass
+
+#![feature(public_private_dependencies)]
+#![deny(external_private_dependency)]
+
+pub struct PublicType {
+    pub field: Option<u8>
+}
+
+fn main() {}