MDEV-8865 Wrong field type or metadata for COALESCE(signed_int_column, unsigned_int_column)

Item_func_hybrid_field_type did not return correct field_type(), cmp_type()
and result_type() in some cases, because cached_result_type and
cached_field_type were set in independent pieces of the code and
did not properly match to each other.
Fix:
- Removing Item_func_hybrid_result_type
- Deriving Item_func_hybrid_field_type directly from Item_func
- Introducing a new class Type_handler which guarantees that
  field_type(), cmp_type() and result_type() are always properly synchronized
  and using the new class in Item_func_hybrid_field_type.

sql/CMakeLists.txt

@@ -135,6 +135,7 @@ SET (SQL_SOURCE
                my_apc.cc my_apc.h mf_iocache_encr.cc
                my_json_writer.cc my_json_writer.h
                rpl_gtid.cc rpl_parallel.cc
+               sql_type.cc sql_type.h
 	       ${WSREP_SOURCES}
                table_cache.cc encryption.cc
                ${CMAKE_CURRENT_BINARY_DIR}/sql_builtin.cc