[libcxxabi] [llvm] Enable demangling of `cp` expression production (PR #114882)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

@ldionne, are there any specific reviewers you would recommend for this? The corresponding Itanium ABI change (linked in the PR description) has landed.

https://github.com/llvm/llvm-project/pull/114882