MDEV-12775 Reuse data type aggregation code for hybrid functions and UNION

Introducing a new class Type_holder (used internally in sql_union.cc),
to reuse exactly the same data type attribute aggregation Type_handler API
for hybrid functions and UNION.

This fixes a number of bugs in UNION:

- MDEV-9495 Wrong field type for a UNION of a signed and an unsigned INT expression
- MDEV-9497 UNION and COALESCE produce different field types for DECIMAL+INT
- MDEV-12594 UNION between fixed length double columns does not always preserve scale
- MDEV-12595 UNION converts INT to BIGINT
- MDEV-12599 UNION is not symmetric when mixing INT and CHAR

Details:

- sql_union.cc: Reusing attribute aggregation for UNION.
  Adding new methods:
  * st_select_lex_unit::join_union_type_handlers()
  * st_select_lex_unit::join_union_type_attributes()
  * st_select_lex_unit::join_union_item_types()
  Removing the old join_types()-based code.

- Changing Type_handler::Item_hybrid_func_fix_attributes()
  to accept "name", Type_handler_hybrid_field_type, Type_all_attributes
  as three separate parameters instead of a single Item_hybrid_func parameter,
  to make it possible to pass both Item_hybrid_func and Type_holder.

- Moving the former special GEOMETRY and ENUM/SET attribute aggregation code
  from Item_type_holder::join_types() to
  * Type_handler_typelib::Item_hybrid_func_fix_attributes().
  * Type_handler_geometry::Item_hybrid_func_fix_attrubutes().
  This makes GEOMETRY/ENUM/SET symmetric with all other data types
  (from the UNION point of view).
  Removing Item_type_holder::join_types() and Item_type_holder::get_full_info().

- Adding new methods into Type_all_attributes:
  * Type_all_attributes::set_geometry_type() and
    Item_hybrid_func::set_geometry_type().
  * Adding Type_all_attributes::get_typelib().
  * Adding Type_all_attributes::set_typelib().

- Adding Type_handler_typelib as a common parent for
  Type_handler_enum and Type_handler_set, to avoid code duplication: they have
  already had two common methods, and we're adding one more shared method.

- Adding Type_all_attributes::set_maybe_null(), as some type handlers
  may want to set maybe_null (e.g. Type_handler_geometry) during data type
  attribute aggregation.

- Changing Type_geometry_attributes() to accept Type_handler
  and Type_all_attributes as two separate parameters, instead
  of a single Item parameter, to make it possible to pass Type_holder.

- Adding Item_args::add_argument().

- Moving Item_args::alloc_arguments() from "protected" to "public".

- Moving Item_type_holder::Item_type_holder() from item.cc to item.h, as
  now it's very simple.
  Btw, this constructor should probably be eventually removed.
  It's now used only in sql_show.cc, which could be modified to use
  Item_return_decimal (for symmetry with Item_return_xxx created for all
  other data types). Or, another option: remove all Item_return_xxx and
  use Item_type_holder for all data types instead.

- storage/tokudb/mysql-test/tokudb/r/type_float.result
  Recording new results (MDEV-12594).

- mysql-test/r/cte_recursive.result
  Recording new results (MDEV-9497)

- mysql-test/r/subselect*.result
  Recording new results (MDEV-12595)

- mysql-test/r/metadata.result
  Recording new results (MDEV-9495)

- mysql-test/r/temp_table.result
  Recording new results (MDEV-12594)

- mysql-test/r/type_float.result
  Recording new results (MDEV-12594)

sql/sql_lex.h

@@ -640,6 +640,14 @@ protected:
   ulonglong found_rows_for_union;
   bool saved_error;
 
+  bool prepare_join(THD *thd, SELECT_LEX *sl, select_result *result,
+                    ulong additional_options,
+                    bool is_union_select);
+  bool join_union_item_types(THD *thd, List<Item> &types, uint count);
+  bool join_union_type_handlers(THD *thd,
+                                class Type_holder *holders, uint count);
+  bool join_union_type_attributes(THD *thd,
+                                  class Type_holder *holders, uint count);
 public:
   // Ensures that at least all members used during cleanup() are initialized.
   st_select_lex_unit()
@@ -749,9 +757,6 @@ public:
 
   /* UNION methods */
   bool prepare(THD *thd, select_result *result, ulong additional_options);
-  bool prepare_join(THD *thd, SELECT_LEX *sl, select_result *result,
-                    ulong additional_options,
-                    bool is_union_select);
   bool optimize();
   bool exec();
   bool exec_recursive();