parser: set `UnionStmt` and `ShowStmt` as readonly (#755)#756

Merged

kennytm merged 1 commit intopingcap:release-3.0from djshow832:cherry_pick_755

Mar 1, 2020

+46-1

Commits on Mar 1, 2020

parser: set UnionStmt and ShowStmt as readonly (pingcap#755 )
djshow832
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