[clang] [clang] Add test for CWG1807 (PR #77637)

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@@ -164,6 +164,8 @@ void A<double, U>::C<V>::f4() {
 }
 } // namespace dr1804
 
+// dr1807 is in separate file
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cor3ntin wrote:

Missing a file?

https://github.com/llvm/llvm-project/pull/77637