cmd/compile: allow unnamed constants to set line number

Fixes #8836.

Change-Id: Idda9f4a987e03b3bdf5e8fdb984fe56d6f84aa59
Reviewed-on: https://go-review.googlesource.com/11672
Reviewed-by: David Chase <drchase@google.com>
diff --git a/src/cmd/compile/internal/gc/subr.go b/src/cmd/compile/internal/gc/subr.go
index beb3c3c..058ae5e 100644
--- a/src/cmd/compile/internal/gc/subr.go
+++ b/src/cmd/compile/internal/gc/subr.go
@@ -231,9 +231,15 @@
 	lno := lineno
 	if n != nil {
 		switch n.Op {
-		case ONAME, OTYPE, OPACK, OLITERAL:
+		case ONAME, OTYPE, OPACK:
 			break
 
+		case OLITERAL:
+			if n.Sym != nil {
+				break
+			}
+			fallthrough
+
 		default:
 			lineno = n.Lineno
 			if lineno == 0 {