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

[Alexander Richardson via cfe-commits]

arichardson wrote:

@ilovepi Thanks for reminding me about this - I should really upstream the remaining changes from https://github.com/CTSRD-CHERI/llvm-project/blob/dev/clang/utils/creduce_crash_testcase.py.

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