cmd/compile: fix two instances of { lineno = ...; yyerror }

Updates #19683

Change-Id: Ic00d5a9807200791cf37553f4f802dbf27beea19
Reviewed-on: https://go-review.googlesource.com/38770
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/walk.go

@@ -49,12 +49,10 @@ func walk(fn *Node) {
 			if defn.Left.Used() {
 				continue
 			}
-			lineno = defn.Left.Pos
-			yyerror("%v declared and not used", ln.Sym)
+			yyerrorl(defn.Left.Pos, "%v declared and not used", ln.Sym)
 			defn.Left.SetUsed(true) // suppress repeats
 		} else {
-			lineno = ln.Pos
-			yyerror("%v declared and not used", ln.Sym)
+			yyerrorl(ln.Pos, "%v declared and not used", ln.Sym)
 		}
 	}