Print hex dump of alloc on reading undef bytes
Here's a small addition I made locally to the UB diagnostics, in case you're interested in it. This PR calls `dump_alloc()` on the relevant allocation if Miri fails on UB due to reading undefined bytes. This came in handy when diagnosing such an issue in a large program using unsafe Rust, in part because it wasn't deterministic enough to use `-Zmiri-track-alloc-id=`. If you'd like to put this behind another -Z flag, let me know.
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