[clang] [CIR] Run LoweringPrepare pass only for non CIR output (PR #154107)

[Henrich Lauko via cfe-commits]

https://github.com/xlauko commented:

Why does this adds `-std=c++20` to tests?

This removes lot of lowering prepare tests, is it intentional? 
Can we add `RUN:` with enabled lowering and keep tests?

https://github.com/llvm/llvm-project/pull/154107