Name the intermediate marshaling type exactly like the original type

The type name shows up in error messages and it's nicer to have the
original struct name there.

genmethod.go

@@ -52,7 +52,7 @@ func genUnmarshalJSON(mtyp *marshalerType) Function {
 		m        = newMarshalMethod(mtyp, true)
 		recv     = m.receiver()
 		input    = Name(m.scope.newIdent("input"))
-		intertyp = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name() + "JSON"))
+		intertyp = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name()))
 		dec      = Name(m.scope.newIdent("dec"))
 		json     = Name(m.scope.parent.packageName("encoding/json"))
 	)
@@ -80,7 +80,7 @@ func genMarshalJSON(mtyp *marshalerType) Function {
 	var (
 		m        = newMarshalMethod(mtyp, false)
 		recv     = m.receiver()
-		intertyp = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name() + "JSON"))
+		intertyp = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name()))
 		enc      = Name(m.scope.newIdent("enc"))
 		json     = Name(m.scope.parent.packageName("encoding/json"))
 	)
@@ -118,7 +118,7 @@ func genUnmarshalLikeYAML(mtyp *marshalerType, name string) Function {
 		m         = newMarshalMethod(mtyp, true)
 		recv      = m.receiver()
 		unmarshal = Name(m.scope.newIdent("unmarshal"))
-		intertyp  = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name() + name))
+		intertyp  = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name()))
 		dec       = Name(m.scope.newIdent("dec"))
 		tag       = strings.ToLower(name)
 	)
@@ -152,7 +152,7 @@ func genMarshalLikeYAML(mtyp *marshalerType, name string) Function {
 	var (
 		m        = newMarshalMethod(mtyp, false)
 		recv     = m.receiver()
-		intertyp = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name() + name))
+		intertyp = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name()))
 		enc      = Name(m.scope.newIdent("enc"))
 		tag      = strings.ToLower(name)
 	)