Forbid the use of #[target_feature] on start

compiler/rustc_hir_analysis/src/errors.rs

@@ -344,6 +344,15 @@ pub(crate) struct StartTrackCaller {
     pub start: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_start_not_target_feature)]
+pub(crate) struct StartTargetFeature {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub start: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_start_not_async, code = "E0752")]
 pub(crate) struct StartAsync {