Convert proc_macro_back_compat lint to an unconditional error.

We still check for the `rental`/`allsorts-rental` crates. But now if
they are detected we just emit a fatal error, instead of emitting a
warning and providing alternative behaviour.

The original "hack" implementing alternative behaviour was added
in #73345.

The lint was added in #83127.

The tracking issue is #83125.

The direct motivation for the change is that providing the alternative
behaviour is interfering with #125174 and follow-on work.

compiler/rustc_expand/src/errors.rs

@@ -440,3 +440,13 @@ pub(crate) struct EmptyDelegationList {
     #[primary_span]
     pub span: Span,
 }
+
+// This used to be the `proc_macro_back_compat` lint (#83125). It was later
+// turned into a hard error.
+#[derive(Diagnostic)]
+#[diag(expand_proc_macro_back_compat)]
+#[note]
+pub struct ProcMacroBackCompat {
+    pub crate_name: String,
+    pub fixed_version: String,
+}