MDEV-16117 SP with a single FOR statement creates but further fails to load

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).

sql/sql_class.h

@@ -4248,16 +4248,8 @@ public:
   void parse_error(const char *err_text, const char *yytext)
   {
     Lex_input_stream *lip= &m_parser_state->m_lip;
-    if (!yytext)
-    {
-      if (lip->has_lookahead())
-        yytext= lip->get_tok_start_prev();
-      else
-        yytext= lip->get_tok_start();
-
-      if (!yytext)
+    if (!yytext && !(yytext= lip->get_tok_start()))
         yytext= "";
-    }
     /* Push an error into the error stack */
     ErrConvString err(yytext, strlen(yytext), variables.character_set_client);
     my_printf_error(ER_PARSE_ERROR,  ER_THD(this, ER_PARSE_ERROR), MYF(0),