Adding:
- new class sp_expr_lex
- new grammar rule expr_lex, which includes both reset_lex()
  and its corresponding restore_lex()

Also:
- Moving a few methods from LEX to sp_expr_lex.
- Moving the code from *.yy to new method sp_expr_lex methods
  sp_repeat_loop_finalize() and sp_if_expr().

This change makes it easier to edit the related grammar
(and makes it easier to unify sql_yacc.yy and sql_yacc_ora.yy later).