Index: llvm/include/llvm/Support/VirtualFileSystem.h
===================================================================
--- llvm/include/llvm/Support/VirtualFileSystem.h
+++ llvm/include/llvm/Support/VirtualFileSystem.h
@@ -296,6 +296,13 @@
   std::error_code makeAbsolute(SmallVectorImpl<char> &Path) const;
 };
 
+/// Interface for virtual file system implementations that have external paths.
+class ExternalFileSystem : public FileSystem {
+public:
+  virtual llvm::ErrorOr<std::string>
+  getExternalPath(const Twine &Path) const = 0;
+};
+
 /// Gets an \p vfs::FileSystem for the 'real' file system, as seen by
 /// the operating system.
 IntrusiveRefCntPtr<FileSystem> getRealFileSystem();
Index: llvm/lib/Support/VirtualFileSystem.cpp
===================================================================
--- llvm/lib/Support/VirtualFileSystem.cpp
+++ llvm/lib/Support/VirtualFileSystem.cpp
@@ -229,7 +229,7 @@
 namespace {
 
 /// The file system according to your operating system.
-class RealFileSystem : public FileSystem {
+class RealFileSystem : public ExternalFileSystem {
 public:
   ErrorOr<Status> status(const Twine &Path) override;
   ErrorOr<std::unique_ptr<File>> openFileForRead(const Twine &Path) override;
@@ -240,6 +240,7 @@
   std::error_code isLocal(const Twine &Path, bool &Result) override;
   std::error_code getRealPath(const Twine &Path,
                               SmallVectorImpl<char> &Output) const override;
+  llvm::ErrorOr<std::string> getExternalPath(const Twine &Path) const override;
 
 private:
   mutable std::mutex CWDMutex;
@@ -303,6 +304,11 @@
   return llvm::sys::fs::real_path(Path, Output);
 }
 
+llvm::ErrorOr<std::string>
+RealFileSystem::getExternalPath(const Twine &Path) const {
+  return Path.str();
+}
+
 IntrusiveRefCntPtr<FileSystem> vfs::getRealFileSystem() {
   static IntrusiveRefCntPtr<FileSystem> FS = new RealFileSystem();
   return FS;
@@ -1112,7 +1118,7 @@
 /// In both cases, the 'name' field may contain multiple path components (e.g.
 /// /path/to/file). However, any directory that contains more than one child
 /// must be uniquely represented by a directory entry.
-class RedirectingFileSystem : public vfs::FileSystem {
+class RedirectingFileSystem : public ExternalFileSystem {
   friend class RedirectingFileSystemParser;
 
   /// The root(s) of the virtual file system.
@@ -1164,14 +1170,14 @@
   /// Looks up the path <tt>[Start, End)</tt> in \p From, possibly
   /// recursing into the contents of \p From if it is a directory.
   ErrorOr<Entry *> lookupPath(sys::path::const_iterator Start,
-                              sys::path::const_iterator End, Entry *From);
+                              sys::path::const_iterator End, Entry *From) const;
 
   /// Get the status of a given an \c Entry.
   ErrorOr<Status> status(const Twine &Path, Entry *E);
 
 public:
   /// Looks up \p Path in \c Roots.
-  ErrorOr<Entry *> lookupPath(const Twine &Path);
+  ErrorOr<Entry *> lookupPath(const Twine &Path) const;
 
   /// Parses \p Buffer, which is expected to be in YAML format and
   /// returns a virtual file system representing its contents.
@@ -1195,6 +1201,8 @@
     return ExternalFS->isLocal(Path, Result);
   }
 
+  llvm::ErrorOr<std::string> getExternalPath(const Twine &Path) const override;
+
   directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override {
     ErrorOr<Entry *> E = lookupPath(Dir);
     if (!E) {
@@ -1726,7 +1734,7 @@
   return FS.release();
 }
 
-ErrorOr<Entry *> RedirectingFileSystem::lookupPath(const Twine &Path_) {
+ErrorOr<Entry *> RedirectingFileSystem::lookupPath(const Twine &Path_) const {
   SmallString<256> Path;
   Path_.toVector(Path);
 
@@ -1757,7 +1765,8 @@
 
 ErrorOr<Entry *>
 RedirectingFileSystem::lookupPath(sys::path::const_iterator Start,
-                                  sys::path::const_iterator End, Entry *From) {
+                                  sys::path::const_iterator End,
+                                  Entry *From) const {
 #ifndef _WIN32
   assert(!isTraversalComponent(*Start) &&
          !isTraversalComponent(From->getName()) &&
@@ -1835,6 +1844,24 @@
   return status(Path, *Result);
 }
 
+ErrorOr<std::string>
+RedirectingFileSystem::getExternalPath(const Twine &Path) const {
+  ErrorOr<Entry *> E = lookupPath(Path);
+  if (!E) {
+    if (IsFallthrough &&
+        E.getError() == llvm::errc::no_such_file_or_directory) {
+      return Path.str();
+    }
+    return E.getError();
+  }
+
+  auto *F = dyn_cast<RedirectingFileEntry>(*E);
+  if (!F)
+    return make_error_code(llvm::errc::not_supported);
+
+  return F->getExternalContentsPath();
+}
+
 namespace {
 
 /// Provide a file wrapper with an overriden status.