Auto merge of rust-lang#54358 - flip1995:beta, r=Manishearth

 [beta] Cancel warning for tool_lints

For the discussion about this, see: rust-lang/rust-clippy#3159

`clippy-preview` is available on stable since 1.29. So when running `cargo +beta clippy` on a crate with `#![allow(clippy_lint)]` a warning is produced, which tells the programmer to change this to `#![allow(clippy::clippy_lint)]`. But since `tool_lints` aren't stable yet, this would require a `#![feature(tool_lints)]`. Features aren't available on stable or beta, so we cannot do this. Even wrapping `cfg_attr(clippy)` around this won't help, since Clippy can also be run from stable or beta toolchain.

r? @Manishearth

src/librustc/lint/levels.rs

@@ -299,7 +299,7 @@ impl<'a> LintLevelsBuilder<'a> {
                                     "change it to",
                                     new_lint_name.to_string(),
                                     Applicability::MachineApplicable,
-                                ).emit();
+                                ).cancel();
 
                                 let src = LintSource::Node(Symbol::intern(&new_lint_name), li.span);
                                 for id in ids {