[clang][cas] Fix incomplete type error in DependencyScanningWorker.h

Fix missing include; the header was not actually standalone due to a an
incomplete type used in IntrusiveRefCntPtr.

(cherry picked from commit 4fd7b98)

Author: benlangmuir

clang/include/clang/Tooling/DependencyScanning/DependencyScanningWorker.h

@@ -15,16 +15,11 @@
 #include "clang/Frontend/PCHContainerOperations.h"
 #include "clang/Tooling/DependencyScanning/DependencyScanningService.h"
 #include "clang/Tooling/DependencyScanning/ModuleDepCollector.h"
+#include "llvm/CAS/CachingOnDiskFileSystem.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
 #include <string>
 
-namespace llvm {
-namespace cas {
-class CachingOnDiskFileSystem;
-}
-} // namespace llvm
-
 namespace clang {
 
 class DependencyOutputOptions;