Fix convenience helper Allow to not SIGSEGV for wrong operands

Now it should just not compile.

PiperOrigin-RevId: 970428735
Change-Id: Ic69469e4d01b4d0c55e53b4b6491af501ac169db

Author: happyCoder92

sandboxed_api/sandbox2/policybuilder.h

@@ -144,9 +144,9 @@ class PolicyBuilder final {
   //
   // Each `type T` is defined in an individual library and individually
   // visibility restricted.
-  template <typename... T>
-  PolicyBuilder& Allow(T... tags) {
-    return (Allow(tags), ...);
+  template <typename TF, typename... T>
+  PolicyBuilder& Allow(TF tag, T... tags) {
+    return Allow(tag), (Allow(tags), ...);
   }
 
   // Disables the use of namespaces.