[VFS] Add a proxy FS that delegates calls to underlying FS by default.

[ioeric]

Summary

This is useful when derived file systems want to override some calls
and still proxy other calls.

[ioeric]

• Add an interface for getting the underlying FS from derived classes.

[ioeric]

• Make constructor explicit.

[sammccall]

include/clang/Basic/VirtualFileSystem.h

346

(nit: drop "virtual" when you have override)

[ioeric]

• remove virtual

include/clang/Basic/VirtualFileSystem.h

public:
   const_iterator overlays_begin() const { return FSList.rbegin(); }
 
   /// 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(); }
 };
 
+/// 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)
+      : FS(std::move(FS)) {}
+
+  llvm::ErrorOr<Status> status(const Twine &Path) override {
+    return FS->status(Path);
+  }
+  llvm::ErrorOr<std::unique_ptr<File>>
+  openFileForRead(const Twine &Path) override {
+    return FS->openFileForRead(Path);
+  }
+  directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override {
+    return FS->dir_begin(Dir, EC);
+  }
+  llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override {
+    return FS->getCurrentWorkingDirectory();
+  }
+  std::error_code setCurrentWorkingDirectory(const Twine &Path) override {
+    return FS->setCurrentWorkingDirectory(Path);
+  }
+  std::error_code getRealPath(const Twine &Path,
+                              SmallVectorImpl<char> &Output) const override {
+    return FS->getRealPath(Path, Output);
+  }
+
+protected:
+  FileSystem &getUnderlyingFS() { return *FS; }
+
+private:
+  IntrusiveRefCntPtr<FileSystem> FS;
+};
+
 namespace detail {
 
 class InMemoryDirectory;
 class InMemoryFile;
 
 } // namespace detail
 
 /// An in-memory file system.