[clang-scan-deps] Introduce the DependencyScanning library with the

thread worker code and better error handling

This commit extracts out the code that will powers the fast scanning
worker into a new file in a new DependencyScanning library. The error
and output handling is improved so that the clients can gather
errors/results from the worker directly.

Differential Revision: https://reviews.llvm.org/D63681

llvm-svn: 364474