Put "checks that detect UB" under their own flag below `debug_assertions`

[RalfJung]

This tracks the implementation of rust-lang/compiler-team#725. See there for all the details.

[saethlin]

Quoting the linked MCP:

Initially it would be unstable as -Zub-checks, and cfg(ub_checks)

#123411 achieves this. The MCP describes a stable flag, and waiting for cfg(overflow_checks) to be stabilized. I do not intend to work on those, so I'm unassigning myself.

[RalfJung]

Closing in favor of #123499.