add Target where clause for on conflict (#4442)

Co-authored-by: Vitaliy Shein <vitaliy.shein@thebricks.com>

clause/on_conflict.go

@@ -3,6 +3,7 @@ package clause
 type OnConflict struct {
 	Columns      []Column
 	Where        Where
+	TargetWhere  Where
 	OnConstraint string
 	DoNothing    bool
 	DoUpdates    Set
@@ -25,6 +26,12 @@ func (onConflict OnConflict) Build(builder Builder) {
 		}
 		builder.WriteString(`) `)
 	}
+	
+	if len(onConflict.TargetWhere.Exprs) > 0 {
+		builder.WriteString(" WHERE ")
+		onConflict.TargetWhere.Build(builder)
+		builder.WriteByte(' ')
+	}
 
 	if onConflict.OnConstraint != "" {
 		builder.WriteString("ON CONSTRAINT ")