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

src/pkg/go/scanner/scanner.go

@@ -150,10 +150,7 @@ func (s *Scanner) interpretLineComment(text []byte) {
 			if line, err := strconv.Atoi(string(text[i+1:])); err == nil && line > 0 {
 				// valid //line filename:line comment
 				filename := string(bytes.TrimSpace(text[len(prefix):i]))
-				if filename == "" {
-					// assume same file as for previous line
-					filename = s.file.Position(s.file.Pos(s.lineOffset)).Filename
-				} else {
+				if filename != "" {
 					filename = filepath.Clean(filename)
 					if !filepath.IsAbs(filename) {
 						// make filename relative to current directory

src/pkg/go/scanner/scanner_test.go

@@ -493,9 +493,8 @@ var segments = []segment{
 	{"\nline3  //line File1.go:100", filepath.Join("dir", "TestLineComments"), 3}, // bad line comment, ignored
 	{"\nline4", filepath.Join("dir", "TestLineComments"), 4},
 	{"\n//line File1.go:100\n  line100", filepath.Join("dir", "File1.go"), 100},
-	{"\n//line :42\n  line1", "dir/File1.go", 42},
+	{"\n//line  \t :42\n  line1", "", 42},
 	{"\n//line File2.go:200\n  line200", filepath.Join("dir", "File2.go"), 200},
-	{"\n//line  \t :123\n  line1", "dir/File2.go", 123},
 	{"\n//line foo\t:42\n  line42", filepath.Join("dir", "foo"), 42},
 	{"\n //line foo:42\n  line44", filepath.Join("dir", "foo"), 44},           // bad line comment, ignored
 	{"\n//line foo 42\n  line46", filepath.Join("dir", "foo"), 46},            // bad line comment, ignored