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

[Stephen Senran Zhang via llvm-commits]

zsrkmyn wrote:

@rjmccall Sorry, I posted a comment but just found that I was wrong, so I deleted it. I believe you were questioning about how long clang *mangler* treated these as substitute candidates, but I was answering about *demangler*.

I haven't worked with `_BitInt` for years, so I'm not sure what's the current status of it -- neither mangler nor demangler. I posted comments just for historical reference. @lfmeadow may know these things much better than me now :-D

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