[clang][deps] add support for dependency scanning with cc1 command line

Allow users to run a dependency scan with a cc1 command line in addition to a driver command line. DependencyScanningAction was already being run with a cc1 command line, but DependencyScanningWorker::computeDependencies assumed that it was always provided a driver command line. Now DependencyScanningWorker::computeDependencies can handle cc1 command lines too.

Reviewed By: jansvoboda11

Differential Revision: https://reviews.llvm.org/D156234