[Cabs2cil] allow unions containing structs with FAMs

src/kernel_internals/typing/cabs2cil.ml

tests/syntax/flexible_array_member.i

tests/syntax/flexible_array_member_invalid1.i

tests/syntax/flexible_array_member_invalid2.i

tests/syntax/flexible_array_member_invalid3.i

tests/syntax/flexible_array_member_invalid4.i

tests/syntax/flexible_array_member_invalid5.i

tests/syntax/oracle/flexible_array_member.res.oracle