Index: include/clang/Tooling/Tooling.h
===================================================================
--- include/clang/Tooling/Tooling.h
+++ include/clang/Tooling/Tooling.h
@@ -296,10 +296,13 @@
   ///        not found in Compilations, it is skipped.
   /// \param PCHContainerOps The PCHContainerOperations for loading and creating
   /// clang modules.
+  /// \param BaseFS Base virtual filesystem used for OverlayFileSystem creation
   ClangTool(const CompilationDatabase &Compilations,
             ArrayRef<std::string> SourcePaths,
             std::shared_ptr<PCHContainerOperations> PCHContainerOps =
-                std::make_shared<PCHContainerOperations>());
+                std::make_shared<PCHContainerOperations>(),
+            IntrusiveRefCntPtr<vfs::FileSystem> BaseFS = 
+                vfs::getRealFileSystem());
 
   ~ClangTool();
 
Index: lib/Tooling/Tooling.cpp
===================================================================
--- lib/Tooling/Tooling.cpp
+++ lib/Tooling/Tooling.cpp
@@ -328,10 +328,11 @@
 
 ClangTool::ClangTool(const CompilationDatabase &Compilations,
                      ArrayRef<std::string> SourcePaths,
-                     std::shared_ptr<PCHContainerOperations> PCHContainerOps)
+                     std::shared_ptr<PCHContainerOperations> PCHContainerOps,
+                     IntrusiveRefCntPtr<vfs::FileSystem> BaseFS)
     : Compilations(Compilations), SourcePaths(SourcePaths),
       PCHContainerOps(std::move(PCHContainerOps)),
-      OverlayFileSystem(new vfs::OverlayFileSystem(vfs::getRealFileSystem())),
+      OverlayFileSystem(new vfs::OverlayFileSystem(BaseFS)),
       InMemoryFileSystem(new vfs::InMemoryFileSystem),
       Files(new FileManager(FileSystemOptions(), OverlayFileSystem)),
       DiagConsumer(nullptr) {