Report disallowed subquery errors as such (instead of parse error)

Subqueries are not allowed in a number of clauses, e.g.:

• EXECUTE IMMEDIATE expr
• EXECUTE .. USING expr
• PREPARE stmt FROM expr
• SELECT .. PROCEDURE ANALYSE(expr, expr)
• PURGE BINARY LOGS BEFORE expr
• HANDLER t1 READ a=expr

If a subquery appears in a disallowed context, a syntax error is reported:

EXECUTE IMMEDIATE (SELECT 'SELECT 1');

ERROR 42000: You have an error in your SQL syntax ... near 'SELECT 'SELECT 1')' at line 1

The grammar uses remember_tok_start to remember the start position of a subquery, to use this position for error reporting:

subselect:

          remember_tok_start

          query_expression

          {

            if (!($$= Lex->parsed_subselect($2, $1)))

              YYABORT;

          }

        ;

Under terms of MDEV-19956 the grammar will be changed significantly. The remember_tok_start has to be removed to avoid shift/reduce and reduce/reduce conflicts.

Let's change the error message from ER_SYNTAX_ERROR to ER_SUBQUERIES_NOT_SUPPORTED.
So it will look as follows:

EXECUTE IMMEDIATE (SELECT 'SELECT 1');

ERROR 42000: EXECUTE IMMEDIATE does not support subqueries or stored functions

Rationale:

• Preserving the old messages is challenging
• The proposed error message is more informative