We have a PR here: https://github.com/gentoo/gentoo/pull/21112
I think with clang 13 you will need that .NET backport these changes https://github.com/dotnet/runtime/pull/60328. I make an issue in `dotnet/source-build` repo https://github.com/dotnet/source-build/issues/2645 where I hope receive reply for this request.
Already in the ::gentoo tree.