[clang] [clang][driver] Expose a frontend option for trap-unreachable (PR #174894)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Thank you for doing this legwork, I wish we could be more aggressive but this at least moves us a little further along. Once you land this it might be nice to leave comments in the more central issues that this is now an option.

https://github.com/llvm/llvm-project/pull/174894