Adjust allow-non-lint-warnings.rs

- Document `allow-non-lint-warnings.rs`
- Move `allow-non-lint-warnings.rs` under `tests/ui/diagnostic-flags/`

Author: jieyouxu

tests/ui/allow-non-lint-warnings.rs

@@ -0,0 +1,20 @@
+// ignore-tidy-linelength
+//! Check that `-A warnings` cli flag applies to *all* warnings, including feature gate warnings.
+//!
+//! This test tries to exercise that by checking that the "empty trait list for derive" warning for
+//! `#[derive()]` is permitted by `-A warnings`, which is a non-lint warning.
+//!
+//! # Relevant context
+//!
+//! - Original impl PR: <https://github.com/rust-lang/rust/pull/21248>.
+//! - RFC 507 "Release channels":
+//!   <https://github.com/rust-lang/rfcs/blob/c017755b9bfa0421570d92ba38082302e0f3ad4f/text/0507-release-channels.md>.
+
+//@ compile-flags: -Awarnings
+//@ check-pass
+
+#[derive()]
+#[derive(Copy, Clone)]
+pub struct Foo;
+
+pub fn main() {}