Pass file name into SubstraitPB import.

[ingomueller-net]

Could we do better here?  even just file name

I think yes, but this would require some code reorganization. A few notes before I forget: I'd need to implement the translation using llvm::SourceMgr and access BufferBuffer:: getBufferIdentifier to get the filename.

Originally posted by @ingomueller-net in #825 (comment)