[clang] [clang][utils] Add auto mode to reduction script (PR #163282)

[Arthur Eubanks via cfe-commits]

https://github.com/aeubanks commented:

we really need to come up with some sort of testing strategy for this, otherwise I think it'll be very hard to maintain this script and make improvements without breaking things (would need some blessed way to crash the compiler, e.g. `#pragma clang __debug crash` crashes the frontend, need to figure out how to crash the middle/backend). I think I'd prefer that we figure that out before making improvements

https://github.com/llvm/llvm-project/pull/163282