[clang] [clang-format] Add ability for clang-format-diff to exit with non-0 status (PR #70883)

[Changyu Bi via cfe-commits]

cbi42 wrote:

Hi - would it make sense to call exit after all the diff_string is written to stdout? This allows a preview of all lines that need to change. Right now, it exits after printing out the changes needed for the first file.

https://github.com/llvm/llvm-project/pull/70883