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

[via cfe-commits]

dyung wrote:

> Any other alternative would be preferable over skipping the test altogether.
> 
> My suggestion is to disable that warning on the command line and then simply delete that expected-warning line.

Oh thanks, I didn't realize this was just testing for a crash rather than conformance type warning. I'll update the test to do as you suggest.

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