Refactor DependencyScanningTool to its own file

Summary:
There's no behavior change - just moving DependencyScanningTool to its own file
since this tool can be reused across both clang-scan-deps binary and an interface
exposed as part of libClang APIs.

Reviewers: arphaman, jkorous, Bigcheese, dexonsmith

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69186

llvm-svn: 375483