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

[Stephen Senran Zhang via llvm-commits]

zsrkmyn wrote:

> Is Clang treating these as substitution candidates, and for how long?

No. And this patch tries to make them as substitution candidates.

I believe this change makes sense as we discussed 2 years ago, but c++abi must be updated first.

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