Add sql_mode specific tokens for the keyword DECLARE

DECLARE has different meanings in the default sql_mode and in sql_mode=ORACLE.

To unify sql_yacc.yy and sql_yacc_ora.yy easier, we'll add sql_mode specific tokens for this keyword and modify the grammar in sql_yacc.yy and sql_yacc_ora.yy accordingly.

Instead of DECLARE_SYM, we'll have separate DECLARE_MARIADB_SYM and DECLARE_ORACLE_SYM.

This will make possible for native and Oracle grammar rules to co-exists in the same *.yy files.

By default, Lex_input_stream::find_keyword() will return DECLARE_MARIADB_SYM, and translate it to DECLARE_ORACLE_SYM in Oracle compatibility mode:

    if (m_thd->variables.sql_mode & MODE_ORACLE)

    {

      switch (symbol->tok) {

        ...

        case DECLARE_MARIADB_SYM:         return DECLARE_ORACLE_SYM;

        ...

      }

    }