Userland has been built with clang since 7.0. The kernels of some Android devices (Pixel 2/3 for example) are built with clang but seems like most still use GCC.

pjmlp said: [clang is] the only choice on Android as of NDK 18

https://news.ycombinator.com/item?id=17617499

Yes.

Now that I think about it, I think Chrome OS is as well?

ChromeOS is based on Gentoo, is it not?

Yes, as far as I'm aware they've been using Gentoo for quite a while.