debug/dwarf: call strings.EqualFold instead of calling Lower twice

Change-Id: I8dcb425c37a067277549ba3bda6a21206459890a
GitHub-Last-Rev: dc51b9b425ca51cd0a4ac494d4c7a3c0e2306951
GitHub-Pull-Request: golang/go#35132
Reviewed-on: https://go-review.googlesource.com/c/go/+/203097
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/dwarf/line.go

@@ -806,7 +806,7 @@ func pathJoin(dirname, filename string) string {
 	// DOS-style path.
 	drive2, filename := splitDrive(filename)
 	if drive2 != "" {
-		if strings.ToLower(drive) != strings.ToLower(drive2) {
+		if !strings.EqualFold(drive, drive2) {
 			// Different drives. There's not much we can
 			// do here, so just ignore the directory.
 			return drive2 + filename