[clang][deps] NFC: Expose more flexible version of `Worker::computeDependencies()`

This patch adds new member function to `DependencyScanningWorker` that allows clients to pass custom `DiagnosticConsumer`, and returns `bool`.

This provides more flexibility compared to the existing version that automatically stringifies diagnostics and returns them in `llvm::Error`.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D134838