[clang-scan-deps] NFC, refactor the DependencyScanningWorker to use a consumer

to report the dependencies to the client

This will allow the scanner to report modular dependencies to the consumer.
This will also allow the scanner to accept regular cc1 clang invocations, e.g.
in an implementation of a libclang C API for clang-scan-deps, that I will add
follow-up patches for in the future.

llvm-svn: 370425