[clang] [clang][NFC] Mark CWG2780 as implemented and add a test (PR #195127)

[Vlad Serebrennikov via cfe-commits]

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@@ -194,6 +194,16 @@ int j = f('a', 2);
 #endif
 } // namespace cwg2770
 
+namespace cwg2780 { // cwg2780: 2.7
+
+void f();
+
+void(&g())(int) {
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Endilll wrote:

Why do we need this complicated function declarator?

https://github.com/llvm/llvm-project/pull/195127