[clang] [clang] Implement CWG2428 "Deprecating a concept" (PR #92295)

[via cfe-commits]

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@@ -45,6 +45,34 @@ void fallthrough(int n) {
 #endif
 }
 
+namespace cwg2428 { // cwg2428: 19
+#if __cplusplus >= 202002L
+template <typename>
+concept C [[deprecated]] = true; // #C
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cor3ntin wrote:

`concept C __attribute__((deprecated)) = true; ` should be ill-formed. that should be easy to test

https://github.com/llvm/llvm-project/pull/92295