clang-format: enforce coding style via Jenkins

- factor out common code to a shared module, and quote path to the
clang-format binary, just in case.
- add a new check-last-commit script that is the CI equivalent of the
exiting git pre-commit hook, but this one handles lack of clang-format
as an error, not as a warning.
- $LODE_HOME/opt/bin is supposed to be in PATH already, so not
mentioning LODE_HOME in ClangFormat::find() explicitly.
- if both COMPILER_PLUGINS and LODE_HOME is set, invoke
solenv/clang-format/check-last-commit as part of 'make check'

To test these changes as part of CI, fix a single style violation in an
already committed, non-blacklisted file.

This depends on the lode.git commit
496123bcae28e06c6d6aeda39a5afd1e1fb1fd98 (utils_Linux: install
clang-format in the Jenkins case, 2017-11-16), otherwise erroring out on
a not installed clang-format as part of the build would be a problem.

Change-Id: Ib3110826194ff78a7f1bed1c3796147e92ccb3ba
Reviewed-on: https://gerrit.libreoffice.org/44939
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

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