[clang] Include expected-no-diagnostics in newly-added test (NFC)

(cherry picked from commit c990d9444350ef583c6d53e84d9c10cebbf65532)

Author: antoniofrighetto

clang/test/SemaCXX/cxx2a-virtual-base-used.cpp

@@ -1,4 +1,6 @@
-// RUN: %clang_cc1 -std=c++20 -verify=cxx20 -triple=x86_64-linux-gnu %s
+// RUN: %clang_cc1 -std=c++2a -verify -triple=x86_64-linux-gnu %s
+// expected-no-diagnostics
+
 // Fixes assertion triggered by https://github.com/llvm/llvm-project/issues/65982
 
 struct A { int y; };