Add support for empty structs (under a switch)

[dc-mak]

So GCC and Clang and the allocator are all I guess impl-defined, so doing different things.

17:18 ➜  cerberus git:(empty-structs) cerberus --nolibc --typecheck-core --exec --batch --switches empty_structs tests/ci/0342-empty_struct.c
Defined {value: "Specified(0)", stdout: "y1: ffffffffffd4, y2: ffffffffffd0, x: ffffffffffd0, y3: ffffffffffcc\n", stderr: "", blocked: "false"}
Time spent: 0.061306 seconds
17:19 ➜  cerberus git:(empty-structs) cerberus-vip --nolibc --typecheck-core --exec --batch --switches empty_structs tests/ci/0342-empty_struct.c
Defined {value: "Specified(0)", stdout: "y1: ffffffffffd4, y2: ffffffffffd0, x: ffffffffffd0, y3: ffffffffffcc\n", stderr: "", blocked: "false"}
Time spent: 0.084719 seconds
17:19 ➜  cerberus git:(empty-structs) gcc --version && gcc tests/ci/0342-empty_struct.c && ./a.out
gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0
...
y1: 7ffcbdf304ec, y2: 7ffcbdf304f0, x: 7ffcbdf304eb, y3: 7ffcbdf304f4
17:19 ➜  cerberus git:(empty-structs) clang-15 --version && clang-15 tests/ci/0342-empty_struct.c && ./a.out
Ubuntu clang version 15.0.7
...
y1: 7ffc74254748, y2: 7ffc74254740, x: 7ffc74254738, y3: 7ffc74254730