[libcxx] [llvm] [libc++][CI] Tests the no RTTI configuration. (PR #65518)

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

LGTM -- this adds coverage for something we claim to support right now but didn't test. I'd be in favour of investigating whether we actually want to support building with RTTI disabled at all (perhaps not), but that would be a separate effort.

https://github.com/llvm/llvm-project/pull/65518