Rollup merge of #130353 - Zalathar:lint-zero, r=jieyouxu

Make some lint doctests compatible with `--stage=0`

Currently, running `x test compiler --stage=0` (with `rust.parallel-compiler=false` to avoid other problems) results in two failures, because these lint doctests aren't compatible with the current stage0 compiler.

In theory, the more “correct” solution would be to wrap the opening triple-backtick line in  `#[cfg_attr(not(bootstrap), doc = "..."]`. However, that causes a few practical problems:
- `tidy` doesn't understand that syntax, and miscounts the number of backticks in the comment block.
- `lint-docs` doesn't understand that syntax, and thinks it's trying to declare the lint name.
- Working around the above problems would cause more work and more confusion for whoever does the next bootstrap beta bump.

So instead this PR adds some bootstrap gates inside the individual doctests, which end up producing the desired behaviour, and are straightforward to remove.

compiler/rustc_lint/src/if_let_rescope.rs

@@ -24,7 +24,7 @@
     /// ### Example
     ///
     /// ```rust,edition2021
-    /// #![feature(if_let_rescope)]
+    /// #![cfg_attr(not(bootstrap), feature(if_let_rescope))] // Simplify this in bootstrap bump.
     /// #![warn(if_let_rescope)]
     /// #![allow(unused_variables)]
     ///

compiler/rustc_lint_defs/src/builtin.rs

@@ -1870,6 +1870,7 @@
     /// ### Example
     ///
     /// ```rust,compile_fail
+    /// # #[cfg_attr(bootstrap)] compile_error!(); // Remove this in bootstrap bump.
     /// #![deny(elided_named_lifetimes)]
     /// struct Foo;
     /// impl Foo {