Rollup merge of #127015 - Urgau:non_local_def-tmp-allow, r=lqd

Switch back `non_local_definitions` lint to allow-by-default

This PR switch back (again) the `non_local_definitions` lint to allow-by-default as T-lang is requesting some (major) changes in the lint inner workings in https://github.com/rust-lang/rust/issues/126768#issuecomment-2192634762.

This PR will need to be beta-backported, as the lint is currently warn-by-default in beta.

compiler/rustc_lint/src/non_local_def.rs

@@ -50,7 +50,7 @@ declare_lint! {
     /// All nested bodies (functions, enum discriminant, array length, consts) (expect for
     /// `const _: Ty = { ... }` in top-level module, which is still undecided) are checked.
     pub NON_LOCAL_DEFINITIONS,
-    Warn,
+    Allow,
     "checks for non-local definitions",
     report_in_external_macro
 }