Revert "[VFS] Use original path when falling back to external FS"

```
/work/omp-vega20-0/openmp-offload-amdgpu-runtime/llvm.src/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&)':
/work/omp-vega20-0/openmp-offload-amdgpu-runtime/llvm.src/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;
          ^
```

This reverts commit c9721756.