[Dependency Scanning] Use a standalone DependencyScanningFilesystem on the scanner, sharing a common status cache from Clang's dependency service cache

lib/AST/ModuleDependencies.cpp

@@ -426,7 +426,6 @@ SwiftDependencyScanningService::SwiftDependencyScanningService() {
       /* CAS (llvm::cas::ObjectStore) */ nullptr,
       /* Cache (llvm::cas::ActionCache) */ nullptr,
       /* SharedFS */ nullptr);
-  SharedFilesystemCache.emplace();
 }
 
 bool