[clang] [clang] Add tests for DRs about complete-class context (PR #77444)

[via cfe-commits]

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@@ -42,6 +42,28 @@ namespace dr1611 { // dr1611: dup 1658
   C c;
 }
 
+namespace dr1626 { // dr1626: no open
+// FIXME: current consensus for CWG2335 is that the examples are well-formed.
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cor3ntin wrote:

It is not clear from https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1626 what the intent for that is.

The minutes are very terse
```
- I agree with you, would be happy to mark as NAD
- Will mark as P1
```


https://github.com/llvm/llvm-project/pull/77444