[clang] Skip test added in #154418 to work when the default is C++20. (PR #154463)

[Matheus Izvekov via cfe-commits]

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mizvekov wrote:

That's also better than disabling the test, but we don't need to test that we produce this warning, this test is not about that. It's a regression test for a crash and the warning is irrelevant.

https://github.com/llvm/llvm-project/pull/154463