[clang] [compiler-rt] [ubsan] Display correct runtime messages for negative _BitInt (PR #93612)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi commented:

Modulo some nits, this seems fine to me. I'll leave approval to one of the code owners, though, as I've had little to do w/ big integer code or UBSAN.

https://github.com/llvm/llvm-project/pull/93612