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

[Paul Kirth via cfe-commits]

ilovepi wrote:

Also, I mentioned several issues in the description, but it wasn't immediately obvious which of those would actually be fixed by this. If folks know which of those (or additional) bugs should be closed with this landed let me know. Though I guess there's an argument to leave them open until we change the default?

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