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

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

This makes sense to me with a rewording of the comment, potentially.

For history, @zsrkmyn originally added this support in a23652f6f9d8bcdeb7eee81c45eaf3ae65cfb3c5.

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