warnings in sql_trigger.cc and sql_view.cc".

According to the current version of C++ standard offsetof() macro
can't be used for non-POD types. So warnings were emitted when we
tried to use this macro for TABLE_LIST and Table_triggers_list
classes. Note that despite of these warnings it was probably safe
thing to do.

This fix tries to circumvent this limitation by implementing
custom version of offsetof() macro to be used with these
classes. This hack should go away once we will refactor
File_parser class.

Alternative approaches such as disabling this warning for
sql_trigger.cc/sql_view.cc or for the whole server were
considered less explicit. Also I was unable to find a way
to disable particular warning for particular _part_ of
file in GCC.


sql/parse_file.h:
  Introduced auxillary macro which can be used instead of offsetof()
  to get offsets of members in class for non-POD types without getting
  warnings (assuming that all instances of the class has same offsets
  for same members).
sql/sql_trigger.cc:
  Use my_offsetof() macro instead of standard offsetof() macro with
  Table_triggers_list class in order to avoid warnings (offsetof()
  cannot be used for non-POD types according to the standard).
sql/sql_view.cc:
  Use my_offsetof() macro instead of standard offsetof() macro with
  TABLE_LIST class in order to avoid warnings (offsetof() cannot
  be used for non-POD types according to the standard).