Provide public getter for OverlayFileSystem

[vladimir.plyashkun]

Summary

This is part of review - https://reviews.llvm.org/D41535

include/clang/Tooling/Tooling.h

public:
 
   /// \brief Returns the file manager used in the tool.
   ///
   /// The file manager is shared between all translation units.
   FileManager &getFiles() { return *Files; }
 
   llvm::ArrayRef<std::string> getSourcePaths() const { return SourcePaths; }
 
+  vfs::OverlayFileSystem &getOverlayFileSystem() const {
+    return *OverlayFileSystem;
+  }
+
 private:
   const CompilationDatabase &Compilations;
   std::vector<std::string> SourcePaths;
   std::shared_ptr<PCHContainerOperations> PCHContainerOps;
 
   llvm::IntrusiveRefCntPtr<vfs::OverlayFileSystem> OverlayFileSystem;
   llvm::IntrusiveRefCntPtr<vfs::InMemoryFileSystem> InMemoryFileSystem;
   llvm::IntrusiveRefCntPtr<FileManager> Files;