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

[Stephen Senran Zhang via llvm-commits]

zsrkmyn wrote:

Thanks for mentioning me here.

But AFAIK, _BitInt and _FloatN were not candidates for substations by c++abi. Are there any updates now? I once made a [proposal](https://github.com/itanium-cxx-abi/cxx-abi/pull/159) to fix the ABI but wasn't merged.

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