Rollup merge of #138084 - nnethercote:workspace-lints, r=jieyouxu

Use workspace lints for crates in `compiler/`

This is nicer and hopefully less error prone than specifying lints via bootstrap.

r? ``@jieyouxu``

compiler/rustc_ast/Cargo.toml

@@ -18,3 +18,6 @@ smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 thin-vec = "0.2.12"
 tracing = "0.1"
 # tidy-alphabetical-end
+
+[lints]
+workspace = true

compiler/rustc_ast/src/lib.rs

@@ -19,7 +19,6 @@
 #![feature(never_type)]
 #![feature(rustdoc_internals)]
 #![feature(stmt_expr_attributes)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub mod util {