[VFS] Add reverse iterator to OverlayFileSystem

[JDevlieghere]

Summary

Add a reverse iterator to the overlay file system. This makes it possible to take overlays from one OverlayFileSystem, and add them to another.

[bruno]

Thanks for working on this. Can you add unit tests for those?

[JDevlieghere]

Add unit test for both iterators

[bruno]

Nice, LGTM!

llvm/include/llvm/Support/VirtualFileSystem.h

public:
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override;
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
   std::error_code isLocal(const Twine &Path, bool &Result) override;
   std::error_code getRealPath(const Twine &Path,
                               SmallVectorImpl<char> &Output) const override;
 
   using iterator = FileSystemList::reverse_iterator;
   using const_iterator = FileSystemList::const_reverse_iterator;
+  using reverse_iterator = FileSystemList::iterator;
+  using const_reverse_iterator = FileSystemList::const_iterator;
 
   /// Get an iterator pointing to the most recently added file system.
   iterator overlays_begin() { return FSList.rbegin(); }
   const_iterator overlays_begin() const { return FSList.rbegin(); }
 
-  /// Get an iterator pointing one-past the least recently added file
-  /// system.
+  /// Get an iterator pointing one-past the least recently added file system.
   iterator overlays_end() { return FSList.rend(); }
   const_iterator overlays_end() const { return FSList.rend(); }
+
+  /// Get an iterator pointing to the least recently added file system.
+  reverse_iterator overlays_rbegin() { return FSList.begin(); }
+  const_reverse_iterator overlays_rbegin() const { return FSList.begin(); }
+
+  /// Get an iterator pointing one-past the most recently added file system.
+  reverse_iterator overlays_rend() { return FSList.end(); }
+  const_reverse_iterator overlays_rend() const { return FSList.end(); }
 };
 
 /// By default, this delegates all calls to the underlying file system. This
 /// is useful when derived file systems want to override some calls and still
 /// proxy other calls.
 class ProxyFileSystem : public FileSystem {
 public:
   explicit ProxyFileSystem(IntrusiveRefCntPtr<FileSystem> FS)