cmd/compile: avoid multiple errors regarding misuse of ... in signatures

Follow-up on #28450 (golang.org/cl/152417).

Updates #28450.
Fixes #29107.

Change-Id: Ib4b4fe582c35315a4f71cf6dbc7f7f2f24b37ec1
Reviewed-on: https://go-review.googlesource.com/c/152758Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -546,12 +546,14 @@ func (p *noder) param(param *syntax.Field, dddOk, final bool) *Node {
 	// rewrite ...T parameter
 	if typ.Op == ODDD {
 		if !dddOk {
-			yyerror("cannot use ... in receiver or result parameter list")
+			// We mark these as syntax errors to get automatic elimination
+			// of multiple such errors per line (see yyerrorl in subr.go).
+			yyerror("syntax error: cannot use ... in receiver or result parameter list")
 		} else if !final {
 			if param.Name == nil {
-				yyerror("cannot use ... with non-final parameter")
+				yyerror("syntax error: cannot use ... with non-final parameter")
 			} else {
-				p.yyerrorpos(param.Name.Pos(), "cannot use ... with non-final parameter %s", param.Name.Value)
+				p.yyerrorpos(param.Name.Pos(), "syntax error: cannot use ... with non-final parameter %s", param.Name.Value)
 			}
 		}
 		typ.Op = OTARRAY

test/fixedbugs/issue28450.go

@@ -6,13 +6,13 @@
 
 package p
 
-func f(a, b, c, d ...int)       {} // ERROR "non-final parameter a" "non-final parameter b" "non-final parameter c"
+func f(a, b, c, d ...int)       {} // ERROR "non-final parameter a"
 func g(a ...int, b ...int)      {} // ERROR "non-final parameter a"
 func h(...int, ...int, float32) {} // ERROR "non-final parameter"
 
 type a func(...float32, ...interface{}) // ERROR "non-final parameter"
 type b interface {
 	f(...int, ...int)                // ERROR "non-final parameter"
-	g(a ...int, b ...int, c float32) // ERROR "non-final parameter a" "non-final parameter b"
+	g(a ...int, b ...int, c float32) // ERROR "non-final parameter a"
 	valid(...int)
 }