[clang] [clang][NFC] Add examples from [dcl.init.aggr] to C++ conformance tests (PR #91435)

[Vlad Serebrennikov via cfe-commits]

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@@ -0,0 +1,294 @@
+// RUN:  %clang_cc1 -std=c++2c -verify %s
+
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Endilll wrote:

Any reference would become outdated the moment this file is touched next time, so I added one to the PR description, so it makes it into the commit message.

https://github.com/llvm/llvm-project/pull/91435