The code in the "sp_tail" rule in sql_yacc.yy always
used YYLIP->get_cpp_tok_start() as the start of the body,
and did not check for possible lookahead which happens
for keywords "FOR", "VALUES" and "WITH" for LALR(2)
resolution in Lex_input_stream::lex_token().

In case of the lookahead token presence,
get_tok_start_prev() should have been used instead
of get_cpp_tok_start() as the beginning of the SP body.

Change summary:

This patch hides the implementation of the lookahead
token completely inside Lex_input_stream.
The users of Lex_input_stream now just get token-by-token
transparently and should not care about lookahead any more.
Now external users of Lex_input_stream
are not aware of the lookahead token at all.

Change details:

- Moving Lex_input_stream::has_lookahead() into the "private" section.

- Removing Lex_input_stream::get_tok_start_prev() and
  Lex_input_stream::get_cpp_start_prev().

- Fixing the external code to call get_tok_start() and get_cpp_tok_start()
  in all places where get_tok_start_prev() and get_cpp_start_prev()
  where used.

- Adding a test for has_lookahead() right inside
  get_tok_start() and get_cpp_tok_start().
  If there is a lookahead token, these methods now
  return the position of the previous token automatically:

   const char *get_tok_start()
   {
     return has_lookahead() ? m_tok_start_prev : m_tok_start;
   }

   const char *get_cpp_tok_start()
   {
    return has_lookahead() ? m_cpp_tok_start_prev : m_cpp_tok_start;
   }

- Fixing the internal code inside Lex_input_stream methods
  to use m_tok_start and m_cpp_tok_start directly,
  instead of calling get_tok_start() and get_cpp_tok_start(),
  to make sure to access to the *current* token position
  (independently of a lookahead token presence).