Rollup merge of #138331 - nnethercote:use-RUSTC_LINT_FLAGS-more, r=onur-ozkan,jieyouxu

Use `RUSTC_LINT_FLAGS` more

An alternative to the failed #138084.

Fixes #138106.

r? `````@jieyouxu`````

compiler/rustc_resolve/src/lib.rs

@@ -20,7 +20,6 @@
 #![feature(let_chains)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::cell::{Cell, RefCell};