For those SP instructions that need to get access to LEX object on execution,
added storing of their original sql expressions inside classes derived
from the class sp_lex_instr.

Stored sql expression is returned by the abstract method
  sp_lex_instr::get_expr_query
redefined in derived classes.

Since an expression constituting a SP instruction could be non valid
SQL statement in general case (not parseable statement), the virtual method
sp_lex_instr::get_query() has been introduced to return a valid string
for a statement that corresponds to the given instruction.

Additionally, introduced the rule remember_start_opt in the grammar.
The new rule intended to get correct position of a current
token taking into attention the fact whether lookahead was done or not.