cmd/compile: use 'not defined' rather than 'unnamed' in error message

A receiver type may have an (alias type) name and thus be 'named'
even though the name doesn't refer to a defined type. Adjust the
error message to make this clearer.

Change-Id: I969bf8d1ba3db8820f67f6ecd6d5cfe564c5b80d
Reviewed-on: https://go-review.googlesource.com/112638Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -881,7 +881,7 @@ func addmethod(msym *types.Sym, t *types.Type, local, nointerface bool) *types.F
 		case t == nil || t.Broke():
 			// rely on typecheck having complained before
 		case t.Sym == nil:
-			yyerror("invalid receiver type %v (%v is an unnamed type)", pa, t)
+			yyerror("invalid receiver type %v (%v is not a defined type)", pa, t)
 		case t.IsPtr():
 			yyerror("invalid receiver type %v (%v is a pointer type)", pa, t)
 		case t.IsInterface():