Allow #[deny(..)] inside #[forbid(..)] as a no-op with a warning

Forbid cannot be overriden. When someome tries to do this anyways,
it results in a hard error. That makes sense.

Except it doesn't, because macros. Macros may reasonably use `#[deny]`
in their expansion to assert
that their expanded code follows the lint. This is doesn't work when the
output gets expanded into a `forbid()` context. This is pretty silly,
since both the macros and the code agree on the lint!

Therefore, we allow `#[deny(..)]`ing a lint that's already forbidden,
keeping the level at forbid.

compiler/rustc_lint_defs/src/builtin.rs

@@ -156,7 +156,7 @@ declare_lint! {
     ///
     /// ```rust
     /// #![forbid(warnings)]
-    /// #![deny(bad_style)]
+    /// #![warn(bad_style)]
     ///
     /// fn main() {}
     /// ```