Revert "Use workspace lints for crates in compiler/ #138084"

Revert <https://github.com/rust-lang/rust/pull/138084> to buy time to
consider options that avoids breaking downstream usages of cargo on
distributed `rustc-src` artifacts, where such cargo invocations fail due
to inability to inherit `lints` from workspace root manifest's
`workspace.lints` (this is only valid for the source rust-lang/rust
workspace, but not really the distributed `rustc-src` artifacts).

This breakage was reported in
<https://github.com/rust-lang/rust/issues/138304>.

This reverts commit 48caf81484, reversing
changes made to c6662879b2.

compiler/rustc_pattern_analysis/src/lib.rs

@@ -6,6 +6,7 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
 #![cfg_attr(feature = "rustc", feature(let_chains))]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub mod constructor;