[llvm] [NFC][cxxabi] Apply `cp-to-llvm.sh` (PR #101970)

[Nico Weber via llvm-commits]

nico wrote:

Looks like that include is from https://reviews.llvm.org/D144252 / a7aade1f36eb60161235b66bca46db12e5326a0c by @philnik777 and wasn't synced either.

@philnik777, we can't have ItaniumDemangle.h include __cxxabi_config.h, see libcxxabi/src/demangle/README.txt. It looks like it's only used to check if we're using clang, which can be done with `#if defined(__clang__)`.

So I think the thing to do is:

1. Make PR to remove that include. I can do that.
2. Update this PR after that PR is in.

https://github.com/llvm/llvm-project/pull/101970