expression: optimize the `Clone` of Constant. (#6262)
expression: optimize the `Clone` of Constant. (#6262)


cosmtrek/tidb,Cofyc/tidb,CodeRushing/tidb,c4pt0r/tidb,XuHuaiyu/tidb,coocood/tidb,pingcap/tidb,qiuyesuifeng/tidb,ming-relax/tidb,ngaut/tidb,wusphinx/tidb,pingcap/tidb,ngaut/tidb,cosmtrek/tidb,kyle-wang/tidb,ngaut/tidb,pingcap/tidb,EvilMcJerkface/tidb,wusphinx/tidb,pingcap/tidb,kyle-wang/tidb,Nify/tidb,Cofyc/tidb,ngaut/tidb,qiuyesuifeng/tidb,Nify/tidb,c4pt0r/tidb,ngaut/tidb,kyle-wang/tidb,c4pt0r/tidb,tiancaiamao/tidb,coocood/tidb,EvilMcJerkface/tidb,mccxj/tidb,pingcap/tidb,ming-relax/tidb,kyle-wang/tidb,mccxj/tidb,pingcap/tidb,tiancaiamao/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,XuHuaiyu/tidb,c4pt0r/tidb,CodeRushing/tidb