struct_assignment: save a lot more information

struct_assignment: save a lot more information

This was not saving nested struct members like foo->bar-baz.  Also unions
members were not saved.

The disadvantage of this change is that it means smatch uses more memory
and is slower.  I have worked pretty hard to cut down on memory usage to
prepare for this change, and I've been running this code myself for a
while and it basically works.  In the old code we saved memory but by
ignoring stuff randomly, the right way is to ignore stuff in a deliberate
way if necessary.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>