builder: use undef bool instead of false, for checked_binop's zombie.

[eddyb]

• fixes Spurious "checked add" errors from false constants, since nightly-2023-03-04. #1024

The actual fix was tiny, but testing required going through some hoops, as the original issue only seemed to reproduce with --release (ironically, given the checked overflow connection).

In the end I went with -Ccodegen-units=1 (to try and force the user's bool constants to overlap the zombie), but I'm not 100% sure that's future-proof. But it did reproduce, i.e. w/o the fix, the test errors with:

error: checked add is not supported yet
   --> /home/eddy/.rustup/toolchains/nightly-2023-03-04-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/arith.rs:104:45
    |
104 |             fn add(self, other: $t) -> $t { self + other }
    |                                             ^^^^^^^^^^^^
    |
    = note: Stack:
            issue_1024::main
            main