Part1: MDEV-12518 Unify sql_yacc.yy and sql_yacc_ora.yy

Mixing %type and %expect declarations:
- sql_mode=ORACLE declarations look like an empty C code section
  inside sql_yacc.yy, consisting of an inactive #ifdef..#endif block.
- sql_mode=DEFAULT declarations look like an empty C code section
  inside sql_yacc_ora.yy, consisting of an inactive #ifdef..#endif block.

Mixing rules:
- Adding a special rule _empty to the shared rule section.
- Changing all instances of /*Empty*/ in sql_mode=DEFAULT and sql_mode=ORACLE
  specific sections to _empty.
- Changing the rest of C style comments /*xxx*/ in
  sql_mode=DEFAULT and sql_mode=ORACLE specific blocks to C++ style: //xxx
- Mixing sql_yacc.yy and sql_yacc_ora.yy, so
  sql_mode=ORACLE specific blocks sit in a comment inside sql_yacc.yy, and
  sql_mode=DEFAULT specific blocks sit in a comment inside sql_yacc_ora.yy.