[exportverilog] Improve location options

We currently print source location information as a comment with line and column info. It would be great to expand this (along with a lowering option) to four different options:

1) None
2) Current comment.
3) `line directives (which cover filename + line)
4) `line and comment (which covers filename+line+col)

Something like `veriloglocationstyle={none,comment,line,both}` would be nice.

-Chris

The issue targets the exportverilog lowering and its source-location output; start by locating the existing comment emission and lowering-option handling. Done means the lowering exposes none, comment, line, and both styles, with the requested filename, line, and column coverage.

Written by the indexing model from the issue text.