[clang] Add tests for CWG issues 6, 212, 232, 2823. (PR #165633)

[Vlad Serebrennikov via cfe-commits]

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@@ -230,6 +230,37 @@ namespace cwg211 { // cwg211: 2.7
   };
 } // namespace cwg211
 
+namespace cwg212 { // cwg212: yes
----------------
Endilll wrote:

```suggestion
namespace cwg212 { // cwg212: 2.7
```
I checked on Compiler Explorer that we exhibit the expected behavior all the way down to 2.7 in both C++98 and C++11 modes

https://github.com/llvm/llvm-project/pull/165633