[clang] [clang][Tests][NFC] Add CWG713 test; add example from CWG1584 test (PR #100747)

[Vlad Serebrennikov via cfe-commits]

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@@ -71,6 +76,17 @@ namespace cwg712 { // cwg712: partial
 #endif
 }
 
+namespace cwg713 { // cwg713: yes
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Endilll wrote:

Please check how just released Clang 19.1-rc1 handles this test, and choose between 19 and 20 accordingly.

https://github.com/llvm/llvm-project/pull/100747