Add diagnostic logging to clangbackend

QTCREATORBUG-20791 has been closed, but a customer just encountered the same issue in Qt Creator 7.0.2:

Clang Code Model: Error: The clangbackend executable "/home/USER/Qt/Tools/QtCreator/libexec/qtcreator/clangbackend" could not be started (timeout after 10000ms).

Manually running the clangbackend executable prints the Usage info (see attached screenshot), which proves that the application is able to launch.

Troubleshooting is difficult as the "qtc.clangbackend.*" logging category doesn't produce anything when the timeout occurs, and clangbackend itself doesn't log anything.

Adding (configurable) logs to clangbackend would be helpful, as QTCREATORBUG-20791 shows that the issue is not an isolated one.