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/structs.h

@@ -1060,4 +1060,29 @@ public:
 };
 
 
+/*
+  A run-time address of an SP variable. Consists of:
+  - The rcontext type (LOCAL, PACKAGE BODY),
+    controlled by m_rcontext_handler
+  - The frame offset
+*/
+class sp_rcontext_addr
+{
+public:
+  sp_rcontext_addr(const class Sp_rcontext_handler *h, uint offset)
+   :m_rcontext_handler(h), m_offset(offset)
+  { }
+  const Sp_rcontext_handler *rcontext_handler() const
+  {
+    return m_rcontext_handler;
+  }
+  uint offset() const
+  {
+    return m_offset;
+  }
+protected:
+  const class Sp_rcontext_handler *m_rcontext_handler;
+  uint m_offset;               ///< Frame offset
+};
+
 #endif /* STRUCTS_INCLUDED */