[clang] [CIR] Change unreachable to diagnostic for ill-equipped clang (PR #152614)

[Arseniy Zaostrovnykh via cfe-commits]

https://github.com/necto commented:

Looks reasonable to me. I have little experience in the frontend configuration code, so I'll let someone else to chip in.

https://github.com/llvm/llvm-project/pull/152614