[NFC] Require asserts for new variadic generics test.

Author: hborla

test/IRGen/variadic_generics.sil

@@ -1,5 +1,8 @@
 // RUN: %target-swift-frontend -emit-ir -primary-file %s -enable-experimental-feature VariadicGenerics | %IRGenFileCheck %s
 
+// Because of -enable-experimental-feature VariadicGenerics
+// REQUIRES: asserts
+
 import Builtin
 import Swift