go/scanner: interpret //line directives sans filename sensibly, second try.

go/scanner: interpret //line directives sans filename sensibly, second try.

A //line directive without a filename now denotes the empty
filename, not the current directory (the Go 1.2 behaviour) nor
the previous //line's filename (the behaviour since CL
86990044).

They should never appear (but they do, e.g. due to a bug in godoc).

Fixes #7765

LGTM=gri, rsc
R=rsc, gri
CC=golang-codereviews
https://golang.org/cl/88160050