[libcxxabi] [llvm] Fix C++ demangling for _BitInt type (PR #143466)

[Louis Dionne via llvm-commits]

https://github.com/ldionne requested changes to this pull request.

I assumed that Clang already produced these manglings. I'd like to have clarity on whether we already do produce these manglings, otherwise this change may indeed never be required. @lfmeadow Can you chime in?

https://github.com/llvm/llvm-project/pull/143466