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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Also, I would think since the resolution is purely editorial (it just changes notes, nothing normative), it would be considered implemented in any clang version?

The question "availability" answers is "What is the first Clang version that exhibits the correct behavior?", where the correct behavior is approximated by the test in our test suite. Because Clang 18 does not pass your CWG713 test, we can't claim that it implements this CWG issue as we understand it.

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