[clang] [clang] Implement CWG2815 (PR #132778)

[Yanzuo Liu via cfe-commits]

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@@ -47,6 +47,28 @@ void f() {
 #endif
 } // namespace cwg2813
 
+namespace cwg2815 { // cwg2815: 21
+#if __cpp_noexcept_function_type >= 201510
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zwuis wrote:

I'm not sure if checking values of feature test macros is better then checking the value of `__cplusplus`, but the rest of the file checks the value of `__cplusplus`. CC @Endilll 

https://github.com/llvm/llvm-project/pull/132778