[clang] [clang] Implement CWG2611 (PR #133747)

[Vlad Serebrennikov via cfe-commits]

================
@@ -35,6 +35,29 @@ namespace std {
   template<typename T> T declval();
 } // namespace std
 
+namespace cwg2611 { // cwg2611: 21
+#if __cpp_fold_expressions >= 201603
----------------
Endilll wrote:

```suggestion
#if __cplusplus >= 201703L
```

https://github.com/llvm/llvm-project/pull/133747