expression: remove the usage of "TypeClass" in "builtin_string.go" (#4573)
expression: remove the usage of "TypeClass" in "builtin_string.go" (#4573)


ngaut/tidb,pingcap/tidb,c4pt0r/tidb,louishust/tidb,tiancaiamao/tidb,kyle-wang/tidb,pingcap/tidb,pingcap/tidb,ngaut/tidb,yuanwhy/tidb,coocood/tidb,ming-relax/tidb,Nify/tidb,zimulala/tidb,ngaut/tidb,Cofyc/tidb,c4pt0r/tidb,Cofyc/tidb,8cbx/tidb,tiancaiamao/tidb,wusphinx/tidb,8cbx/tidb,pingcap/tidb,qiuyesuifeng/tidb,c4pt0r/tidb,Nify/tidb,ming-relax/tidb,kyle-wang/tidb,qiuyesuifeng/tidb,c4pt0r/tidb,zimulala/tidb,kyle-wang/tidb,kyle-wang/tidb,EvilMcJerkface/tidb,spongedu/tidb,pingcap/tidb,cosmtrek/tidb,XuHuaiyu/tidb,ngaut/tidb,cosmtrek/tidb,kyle-wang/tidb,XuHuaiyu/tidb,coocood/tidb,CodeRushing/tidb,yuanwhy/tidb,spongedu/tidb,ngaut/tidb,louishust/tidb,mccxj/tidb,yangwenmai/tidb,CodeRushing/tidb,pingcap/tidb,mccxj/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,wusphinx/tidb,yangwenmai/tidb,EvilMcJerkface/tidb,kyle-wang/tidb