reland: [VFS] Use original path when falling back to external FS

This reverts commit f0cf544d.

Just a small change to fix:

```
/home/buildbot/as-builder-4/llvm-clang-x86_64-expensive-checks-ubuntu/llvm-project/llvm/lib/Support/VirtualFileSystem.cpp: In static member function ‘static llvm::ErrorOr<std::unique_ptr<llvm::vfs::File> > llvm::vfs::File::getWithPath(llvm::ErrorOr<std::unique_ptr<llvm::vfs::File> >, const llvm::Twine&)’:
/home/buildbot/as-builder-4/llvm-clang-x86_64-expensive-checks-ubuntu/llvm-project/llvm/lib/Support/VirtualFileSystem.cpp:2084:10: error: could not convert ‘F’ from ‘std::unique_ptr<llvm::vfs::File>’ to ‘llvm::ErrorOr<std::unique_ptr<llvm::vfs::File> >’
   return F;
          ^
```

Differential Revision: https://reviews.llvm.org/D113832