Auto merge of #95149 - cjgillot:once-diag, r=estebank

Remove `Session::one_time_diagnostic`

This is untracked mutable state, which modified the behaviour of queries.
It was used for 2 things: some full-blown errors, but mostly for lint declaration notes ("the lint level is defined here" notes).

It is replaced by the diagnostic deduplication infra which already exists in the diagnostic emitter.
A new diagnostic level `OnceNote` is introduced specifically for lint notes, to deduplicate subdiagnostics.

As a drive-by, diagnostic emission takes a `&mut` to allow dropping the `SubDiagnostic`s.

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -255,8 +255,8 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         // "secondary" errors if they occurred.
         let secondary_errors = mem::take(&mut self.secondary_errors);
         if self.error_emitted.is_none() {
-            for error in secondary_errors {
-                self.tcx.sess.diagnostic().emit_diagnostic(&error);
+            for mut error in secondary_errors {
+                self.tcx.sess.diagnostic().emit_diagnostic(&mut error);
             }
         } else {
             assert!(self.tcx.sess.has_errors().is_some());