A cleanup for 84c55a5668db582aa92dd2ccf076fbb783894b12 (that implemented cursor FOR loops earlier):

MDEV-10581 sql_mode=ORACLE: Explicit cursor FOR LOOP
  MDEV-12098 sql_mode=ORACLE: Implicit cursor FOR loop

Cleanup changes:
- Removing sp_lex_cursor::m_cursor_name
- Adding sp_instr_cursor_copy_struct::m_cursor (the cursor global index)
- Fixing sp_instr_cursor_copy_struct::print() to access to the cursor
  name using m_ctx and m_cursor (like other cursor related instructions do)
  instead of m_cursor_name.

This change is needed to unify sp_assignment_lex and sp_cursor_lex later,
to fix this problem easier:
 MDEV-16558 Parenthesized expression does not work as a lower FOR loop bound

sql/sp_head.cc

@@ -4506,7 +4506,7 @@ void
 sp_instr_cursor_copy_struct::print(String *str)
 {
   sp_variable *var= m_ctx->find_variable(m_var);
-  const LEX_CSTRING *name= m_lex_keeper.cursor_name();
+  const LEX_CSTRING *name= m_ctx->find_cursor(m_cursor);
   str->append(STRING_WITH_LEN("cursor_copy_struct "));
   str->append(name);
   str->append(' ');
@@ -5001,7 +5001,8 @@ bool sp_head::add_for_loop_open_cursor(THD *thd, sp_pcontext *spcont,
 
   sp_instr *instr_copy_struct=
     new (thd->mem_root) sp_instr_cursor_copy_struct(instructions(),
-                                                    spcont, pcursor->lex(),
+                                                    spcont, coffset,
+                                                    pcursor->lex(),
                                                     index->offset);
   if (instr_copy_struct == NULL || add_instr(instr_copy_struct))
     return true;

sql/sp_head.h

@@ -1004,12 +1004,10 @@ public:
 
 class sp_lex_cursor: public sp_lex_local, public Query_arena
 {
-  LEX_CSTRING m_cursor_name;
 public:
   sp_lex_cursor(THD *thd, const LEX *oldlex, MEM_ROOT *mem_root_arg)
    :sp_lex_local(thd, oldlex),
-    Query_arena(mem_root_arg, STMT_INITIALIZED_FOR_SP),
-    m_cursor_name(null_clex_str)
+    Query_arena(mem_root_arg, STMT_INITIALIZED_FOR_SP)
   { }
   sp_lex_cursor(THD *thd, const LEX *oldlex)
    :sp_lex_local(thd, oldlex),
@@ -1037,8 +1035,6 @@ public:
     thd->free_list= NULL;
     return false;
   }
-  const LEX_CSTRING *cursor_name() const { return &m_cursor_name; }
-  void set_cursor_name(const LEX_CSTRING *name) { m_cursor_name= *name; }
 };
 
 
@@ -1213,10 +1209,6 @@ public:
     m_lex->safe_to_cache_query= 0;
   }
 
-  const LEX_CSTRING *cursor_name() const
-  {
-    return m_lex->cursor_name();
-  }
 private:
 
   LEX *m_lex;
@@ -1864,11 +1856,14 @@ class sp_instr_cursor_copy_struct: public sp_instr
   sp_instr_cursor_copy_struct(const sp_instr_cursor_copy_struct &);
   void operator=(sp_instr_cursor_copy_struct &);
   sp_lex_keeper m_lex_keeper;
+  uint m_cursor;
   uint m_var;
 public:
-  sp_instr_cursor_copy_struct(uint ip, sp_pcontext *ctx,
+  sp_instr_cursor_copy_struct(uint ip, sp_pcontext *ctx, uint coffs,
                               sp_lex_cursor *lex, uint voffs)
-    : sp_instr(ip, ctx), m_lex_keeper(lex, FALSE), m_var(voffs)
+    : sp_instr(ip, ctx), m_lex_keeper(lex, FALSE),
+      m_cursor(coffs),
+      m_var(voffs)
   {}
   virtual ~sp_instr_cursor_copy_struct()
   {}

sql/sql_lex.cc

@@ -5484,7 +5484,8 @@ LEX::sp_variable_declarations_cursor_rowtype_finalize(THD *thd, int nvars,
     spvar->field_def.set_cursor_rowtype_ref(offset);
     sp_instr_cursor_copy_struct *instr=
       new (thd->mem_root) sp_instr_cursor_copy_struct(sphead->instructions(),
-                                                      spcont, pcursor->lex(),
+                                                      spcont, offset,
+                                                      pcursor->lex(),
                                                       spvar->offset);
     if (instr == NULL || sphead->add_instr(instr))
      return true;
@@ -5925,7 +5926,6 @@ bool LEX::sp_declare_cursor(THD *thd, const LEX_CSTRING *name,
     my_error(ER_SP_DUP_CURS, MYF(0), name->str);
     return true;
   }
-  cursor_stmt->set_cursor_name(name);
 
   if (unlikely(spcont->add_cursor(name, param_ctx, cursor_stmt)))
     return true;

sql/sql_lex.h

@@ -3177,8 +3177,6 @@ public:
     return NULL;
   }
 
-  virtual const LEX_CSTRING *cursor_name() const { return &null_clex_str; }
-
   void start(THD *thd);
 
   inline bool is_ps_or_view_context_analysis()