MDEV-22164 without validation for exchange partition/convert in

1. WITHOUT/WITH VALIDATION may be added to EXCHANGE PARTITION or CONVERT TABLE:

  alter table tp exchange partition p1 with table t with validation;
  alter table tp exchange partition p1 with table t; -- same as with validation
  alter table tp exchange partition p1 with table t without validation;

2. Optional THAN keyword for RANGE partitioning. Normally you type:

  create table tp (a int primary key) partition by range (a) (
    partition p0 values less than (100),
    partition p1 values less than maxvalue);

  Now you may type (PARTITION keyword is also optional):

  create table tp (a int primary key) partition by range (a) (
    p0 values less (100),
    p1 values less maxvalue);

sql/sql_lex.h

@@ -3440,6 +3440,7 @@ public:
   bool default_used:1;    /* using default() function */
   bool with_rownum:1;     /* Using rownum() function */
   bool is_lex_started:1;  /* If lex_start() did run. For debugging. */
+  bool without_validation:1; /* exchange or convert partition WITHOUT VALIDATION */
 
   /*
     This variable is used in post-parse stage to declare that sum-functions,