[clang] Load `-fembed-offload-object=` through the VFS (#160906)
This PR loads the path from `-fembed-offload-object=<path>` through the
VFS rather than going straight to the real file system. This matches the
behavior of other input files of the compiler. This technically changes
behavior in that `-fembed-offload-object=-` no longer loads the file
from stdin, but I don't think that was the intention of the original
code anyways.