back.rtlil: attach source locations to switches, not processes.

back.rtlil: attach source locations to switches, not processes.

This effectively reverts and reimplements half of commit 82903e49.
I was confused and did not realize that RTLIL does, in fact, have
attributes on switches.

After this commit, processes no longer have any source locations.
This is semantically fine, as the processes we emit are purely
artificial (because of LHS grouping), but I have not checked how
downstream tooling handles this.