add comment and test: we do not do value-based reasoning for promotion of unions

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

@@ -284,6 +284,7 @@ where
                 if Q::in_adt_inherently(cx, def, args) {
                     return true;
                 }
+                // Don't do any value-based reasoning for unions.
                 if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
                     return true;
                 }