planner: update the syntax of drop hypo index (#44944)

close pingcap/tidb#43817

parser/ast/ddl.go

@@ -1830,6 +1830,7 @@ type DropIndexStmt struct {
 	IndexName string
 	Table     *TableName
 	LockAlg   *IndexLockAndAlgorithm
+	IsHypo    bool // whether this operation is for a hypothetical index.
 }
 
 // Restore implements Node interface.

parser/parser.y

@@ -5023,6 +5023,10 @@ DropIndexStmt:
 		}
 		$$ = &ast.DropIndexStmt{IfExists: $3.(bool), IndexName: $4, Table: $6.(*ast.TableName), LockAlg: indexLockAndAlgorithm}
 	}
+|	"DROP" "HYPO" "INDEX" IfExists Identifier "ON" TableName
+	{
+		$$ = &ast.DropIndexStmt{IfExists: $4.(bool), IndexName: $5, Table: $7.(*ast.TableName), IsHypo: true}
+	}
 
 DropTableStmt:
 	"DROP" OptTemporary TableOrTables IfExists TableNameList RestrictOrCascadeOpt