Index: llvm/include/llvm/Support/FileCollector.h
===================================================================
--- llvm/include/llvm/Support/FileCollector.h
+++ llvm/include/llvm/Support/FileCollector.h
@@ -20,6 +20,35 @@
 class FileCollectorFileSystem;
 class Twine;
 
+class FileCollectorBase {
+public:
+  FileCollectorBase();
+  virtual ~FileCollectorBase();
+
+  void addFile(const Twine &file);
+  void addDirectory(const Twine &Dir);
+
+protected:
+  bool markAsSeen(StringRef Path) {
+    if (Path.empty())
+      return false;
+    return Seen.insert(Path).second;
+  }
+
+  virtual void addFileImpl(StringRef SrcPath) = 0;
+
+  virtual llvm::vfs::directory_iterator
+  addDirectoryImpl(const llvm::Twine &Dir,
+                   IntrusiveRefCntPtr<vfs::FileSystem> FS,
+                   std::error_code &EC) = 0;
+
+  /// Synchronizes access to internal data structures.
+  std::mutex Mutex;
+
+  /// Tracks already seen files so they can be skipped.
+  StringSet<> Seen;
+};
+
 /// Captures file system interaction and generates data to be later replayed
 /// with the RedirectingFileSystem.
 ///
@@ -38,16 +67,13 @@
 ///
 /// In order to preserve the relative topology of files we use their real paths
 /// as relative paths inside of the Root.
-class FileCollector {
+class FileCollector : public FileCollectorBase {
 public:
   /// \p Root is the directory where collected files are will be stored.
   /// \p OverlayRoot is VFS mapping root.
   /// \p Root directory gets created in copyFiles unless it already exists.
   FileCollector(std::string Root, std::string OverlayRoot);
 
-  void addFile(const Twine &file);
-  void addDirectory(const Twine &Dir);
-
   /// Write the yaml mapping (for the VFS) to the given file.
   std::error_code writeMapping(StringRef MappingFile);
 
@@ -67,12 +93,6 @@
 private:
   friend FileCollectorFileSystem;
 
-  bool markAsSeen(StringRef Path) {
-    if (Path.empty())
-      return false;
-    return Seen.insert(Path).second;
-  }
-
   bool getRealPath(StringRef SrcPath, SmallVectorImpl<char> &Result);
 
   void addFileToMapping(StringRef VirtualPath, StringRef RealPath) {
@@ -83,14 +103,12 @@
   }
 
 protected:
-  void addFileImpl(StringRef SrcPath);
+  void addFileImpl(StringRef SrcPath) override;
 
   llvm::vfs::directory_iterator
   addDirectoryImpl(const llvm::Twine &Dir,
-                   IntrusiveRefCntPtr<vfs::FileSystem> FS, std::error_code &EC);
-
-  /// Synchronizes access to Seen, VFSWriter and SymlinkMap.
-  std::mutex Mutex;
+                   IntrusiveRefCntPtr<vfs::FileSystem> FS,
+                   std::error_code &EC) override;
 
   /// The directory where collected files are copied to in copyFiles().
   const std::string Root;
@@ -98,9 +116,6 @@
   /// The root directory where the VFS overlay lives.
   const std::string OverlayRoot;
 
-  /// Tracks already seen files so they can be skipped.
-  StringSet<> Seen;
-
   /// The yaml mapping writer.
   vfs::YAMLVFSWriter VFSWriter;
 
Index: llvm/lib/Support/FileCollector.cpp
===================================================================
--- llvm/lib/Support/FileCollector.cpp
+++ llvm/lib/Support/FileCollector.cpp
@@ -15,6 +15,22 @@
 
 using namespace llvm;
 
+FileCollectorBase::FileCollectorBase() = default;
+FileCollectorBase::~FileCollectorBase() = default;
+
+void FileCollectorBase::addFile(const Twine &File) {
+  std::lock_guard<std::mutex> lock(Mutex);
+  std::string FileStr = File.str();
+  if (markAsSeen(FileStr))
+    addFileImpl(FileStr);
+}
+
+void FileCollectorBase::addDirectory(const Twine &Dir) {
+  assert(sys::fs::is_directory(Dir));
+  std::error_code EC;
+  addDirectoryImpl(Dir, vfs::getRealFileSystem(), EC);
+}
+
 static bool isCaseSensitivePath(StringRef Path) {
   SmallString<256> TmpDest = Path, UpperDest, RealDest;
 
@@ -61,19 +77,6 @@
   return true;
 }
 
-void FileCollector::addFile(const Twine &File) {
-  std::lock_guard<std::mutex> lock(Mutex);
-  std::string FileStr = File.str();
-  if (markAsSeen(FileStr))
-    addFileImpl(FileStr);
-}
-
-void FileCollector::addDirectory(const Twine &Dir) {
-  assert(sys::fs::is_directory(Dir));
-  std::error_code EC;
-  addDirectoryImpl(Dir, vfs::getRealFileSystem(), EC);
-}
-
 void FileCollector::addFileImpl(StringRef SrcPath) {
   // We need an absolute src path to append to the root.
   SmallString<256> AbsoluteSrc = SrcPath;