value_sett::make_member is unsound wrt struct casts

[smowton]

Faced with an expression like ((struct X)some_struct_y).a, value-set's make_member function will reduce that unsoundly to some_struct_y.a, here: https://github.com/diffblue/cbmc/blob/develop/src/pointer-analysis/value_set.cpp#L1654

Clearly that only works when the two structs happen to have the same layout and names.

Expressions like that are hard to get from C, but value_set_dereferencet will produce them when it different struct types are aliased.

#3562 may offer a solution, by soundly simplifying member-of-cast-of-struct-typed-expression when possible.