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_mir_transform/src/check_unsafety.rs

@@ -554,7 +554,6 @@ fn report_unused_unsafe(tcx: TyCtxt<'_>, kind: UnusedUnsafe, id: HirId) {
                     tcx.lint_level_at_node(UNSAFE_OP_IN_UNSAFE_FN, usage_lint_root);
                 assert_eq!(level, Level::Allow);
                 lint::explain_lint_level_source(
-                    tcx.sess,
                     UNSAFE_OP_IN_UNSAFE_FN,
                     Level::Allow,
                     source,