Also the Kernel used in the new Valve Steam deck.

Can clang compile vanilla kernel without Google's forks?

AFAIK it can. They actually spent quite a considerable amount of effort implementing all of the gcc extensions to do so. There is even documentation for that: https://www.kernel.org/doc/html/latest/kbuild/llvm.html

Apparently OpenMandriva also uses a clang-built kernel, presumably not a Google fork. I have no personal knowledge about OpenMandriva though.

Do you consider the ClangBuiltLinux project [0] a Google fork?

[0] https://clangbuiltlinux.github.io/

Clang 9.0+ & Linux 5.3+ work for x86_64; I believe it's been possible to compile arm64 for longer.