[clang] [Clang] Backport P2741R3 (static_assert with user-generated message) to C++11 (PR #102044)

[Mital Ashok via cfe-commits]

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@@ -134,7 +134,7 @@ void test() {
 }
 
 namespace cwg2798 { // cwg2798: 17
-#if __cpp_static_assert >= 202306
+#if __cplusplus > 202302L
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MitalAshok wrote:

I guess this is one of the potentially-breaking changes that comes with changing the feature test macro. I think this is incorrect code and should have been `__cpp_static_assert >= 202306L && __cpp_constexpr >= 201907L`.

For DR testing, I don't think we check extensions, which is why I changed this to check `__cplusplus`. @Endilll is that right?

https://github.com/llvm/llvm-project/pull/102044