[VFS] Add working directories to every virtual file system.

For RealFileSystem this is getcwd()/chdir(), the synthetic file systems can
make up one for themselves. OverlayFileSystem now synchronizes the working
directories when a new FS is added to the overlay or the overlay working
directory is set. This allows purely artificial file systems that have zero
ties to the underlying disks.

Differential Revision: http://reviews.llvm.org/D13430

llvm-svn: 249316

clang/unittests/Basic/VirtualFileSystemTest.cpp

@@ -43,6 +43,12 @@ public:
   openFileForRead(const Twine &Path) override {
     llvm_unreachable("unimplemented");
   }
+  llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override {
+    return std::string();
+  }
+  std::error_code setCurrentWorkingDirectory(const Twine &Path) override {
+    return std::error_code();
+  }
 
   struct DirIterImpl : public clang::vfs::detail::DirIterImpl {
     std::map<std::string, vfs::Status> &FilesAndDirs;