[clang][Dependency Scanning][NFC] Move DependencyScanningAction to its own header and source files (llvm#160795)

This is the first of three PRs to land
llvm#160207 in smaller pieces.

This PR is an NFC. It moves `DependencyScanningAction` to its own source
file, so we can later implement a `CompilerInstanceWithContext` in the
new file.

Part of work for rdar://136303612.

Author: qiongsiwu

clang/lib/Tooling/DependencyScanning/CMakeLists.txt

@@ -10,6 +10,7 @@ add_clang_library(clangDependencyScanning
   DependencyScanningService.cpp
   DependencyScanningWorker.cpp
   DependencyScanningTool.cpp
+  DependencyScannerImpl.cpp
   InProcessModuleCache.cpp
   ModuleDepCollector.cpp