Don't crash if a builtin fn is declared with positional parameters. (#5444)

Crash discovered by fuzzer.

toolchain/check/handle_function.cpp

@@ -681,6 +681,11 @@ static auto IsValidBuiltinDeclaration(Context& context,
                                       const SemIR::Function& function,
                                       SemIR::BuiltinFunctionKind builtin_kind)
     -> bool {
+  if (!function.call_params_id.has_value()) {
+    // For now, we have no builtins that support positional parameters.
+    return false;
+  }
+
   // Find the list of call parameters other than the implicit return slot.
   auto call_params = context.inst_blocks().Get(function.call_params_id);
   if (function.return_slot_pattern_id.has_value()) {