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.