[clang] Fix test added in #154418 to work when the default is C++20. (PR #154463)

[via cfe-commits]

https://github.com/dyung created https://github.com/llvm/llvm-project/pull/154463

The test added for #154418 specifically tests for a warning that the compiler should emit informing the user that a feature is a C++20 extension, but the compiler does not emit that when the default is C++20, so we should skip that test. This change checks whether the C++ default standard is less than C++20, and if so, skips the test.

>From 0a7441df71462230032666d0a89ff736af675218 Mon Sep 17 00:00:00 2001
From: Douglas Yung <dyung+github at fastmail.fm>
Date: Tue, 19 Aug 2025 23:44:47 -0400
Subject: [PATCH] Fix test added in #154418 to work when the default is C++20.

---
 clang/test/SemaTemplate/nested-name-spec-template.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/test/SemaTemplate/nested-name-spec-template.cpp b/clang/test/SemaTemplate/nested-name-spec-template.cpp
index c99e1eed456cc..833f76218212a 100644
--- a/clang/test/SemaTemplate/nested-name-spec-template.cpp
+++ b/clang/test/SemaTemplate/nested-name-spec-template.cpp
@@ -168,7 +168,7 @@ namespace unresolved_using {
   template struct C<int>;
 } // namespace unresolved_using
 
-#if __cplusplus >= 201703L
+#if (__cplusplus >= 201703L && __cplusplus < 202002L)
 namespace SubstTemplateTypeParmPackType {
   template <int...> struct A {};