[clang-format] Add --fail-on-incomplete-format. (#84346)

At the moment clang-format will return exit code 0 on incomplete
results. In scripts it would sometimes be useful if clang-format would
instead fail in those cases, signalling that there was something wrong
with the code being formatted.

---------

Co-authored-by: Björn Schäpers <github@hazardy.de>
Co-authored-by: Owen Pan <owenpiano@gmail.com>