MDEV-36047 Package body variables are not allowed as FETCH targets

It was not possible to use a package body variable as a
fetch target:

CREATE PACKAGE BODY pkg AS
  vc INT := 0;
  FUNCTION f1 RETURN INT AS
    CURSOR cur IS SELECT 1 AS c FROM DUAL;
  BEGIN
    OPEN cur;
    FETCH cur INTO vc; -- this returned "Undeclared variable: vc" error.
    CLOSE cur;
    RETURN vc;
  END;
END;

FETCH assumed that all fetch targets reside of the same sp_rcontext
instance with the cursor. This patch fixes the problem.
Now a cursor and its fetch target can reside in different sp_rcontext
instances.

Details:

- Adding a helper class sp_rcontext_addr
  (a combination of Sp_rcontext_handler pointer and an offset in the rcontext)

- Adding a new class sp_fetch_target deriving from sp_rcontext_addr.
  Fetch targets in "FETCH cur INTO target1, target2 ..." are now collected
  into this structure instead of sp_variable.
  sp_variable cannot be used any more to store fetch targets,
  because it does not have a pointer to Sp_rcontext_handler
  (it only has the current rcontext offset).

- Removing members sp_instr_set members m_rcontext_handler and m_offset.
  Deriving sp_instr_set from sp_rcontext_addr instead.

- Renaming sp_instr_cfetch member  "List<sp_variable> m_varlist"
  to "List<sp_fetch_target> m_fetch_target_list".

- Fixing LEX::sp_add_cfetch() to return the pointer to the
  created sp_fetch_target instance (instead of returning bool).
  This helps to make the grammar in sql_yacc.c simpler

- Renaming LEX::sp_add_cfetch() to LEX::sp_add_instr_cfetch(),
  as `if(sp_add_cfetch())` changed its meaning to the opposite,
  to avoid automatic wrong merge from earlier versions.

- Chaning the "List<sp_variable> *vars" parameter to sp_cursor::fetch
  to have the data type "List<sp_fetch_target> *".

- Changing the data type of "List<sp_variable> &vars" in
  sp_cursor::Select_fetch_into_spvars::send_data_to_variable_list()
  to "List<sp_fetch_target> &".

- Adding THD helper methods get_rcontext() and get_variable().

- Moving the code from sql_yacc.yy into a new LEX method
  LEX::make_fetch_target().

- Simplifying the grammar in sql_yacc.yy using the new LEX method.
  Changing the data type of the bison rule sp_fetch_list from "void"
  to "List<sp_fetch_target> *".

sql/sql_class.h

@@ -3985,6 +3985,9 @@ public:
 
   sp_rcontext *spcont;		// SP runtime context
 
+  sp_rcontext *get_rcontext(const sp_rcontext_addr &addr);
+  Item_field *get_variable(const sp_rcontext_addr &addr);
+
   /** number of name_const() substitutions, see sp_head.cc:subst_spvars() */
   uint       query_name_consts;
 
@@ -6330,10 +6333,11 @@ private:
   /// FETCH <cname> INTO <varlist>.
   class Select_fetch_into_spvars: public select_result_interceptor
   {
-    List<sp_variable> *spvar_list;
+    List<sp_fetch_target> *m_fetch_target_list;
     uint field_count;
     bool m_view_structure_only;
-    bool send_data_to_variable_list(List<sp_variable> &vars, List<Item> &items);
+    bool send_data_to_variable_list(List<sp_fetch_target> &vars,
+                                    List<Item> &items);
   public:
     Select_fetch_into_spvars(THD *thd_arg, bool view_structure_only)
      :select_result_interceptor(thd_arg),
@@ -6342,11 +6346,14 @@ private:
     void reset(THD *thd_arg)
     {
       select_result_interceptor::reinit(thd_arg);
-      spvar_list= NULL;
+      m_fetch_target_list= NULL;
       field_count= 0;
     }
     uint get_field_count() { return field_count; }
-    void set_spvar_list(List<sp_variable> *vars) { spvar_list= vars; }
+    void set_spvar_list(List<sp_fetch_target> *vars)
+    {
+      m_fetch_target_list= vars;
+    }
 
     bool send_eof() override { return FALSE; }
     int send_data(List<Item> &items) override;
@@ -6376,7 +6383,7 @@ public:
   my_bool is_open()
   { return MY_TEST(server_side_cursor); }
 
-  int fetch(THD *, List<sp_variable> *vars, bool error_on_no_data);
+  int fetch(THD *, List<sp_fetch_target> *vars, bool error_on_no_data);
 
   bool export_structure(THD *thd, Row_definition_list *list);