cmd/compile/internal/syntax: implement //line :line:col handling

cmd/compile/internal/syntax: implement //line :line:col handling

For line directives which have a line and a column number,
an omitted filename means that the filename has not changed
(per the issue below).

For line directives w/o a column number, an omitted filename
means the empty filename (to preserve the existing behavior).

For #22662.

Change-Id: I32cd9037550485da5445a34bb104706eccce1df1
Reviewed-on: https://go-review.googlesource.com/96476
Reviewed-by: Matthew Dempsky <mdempsky@google.com>