There is significant investment in building Linux with Clang so I wouldn't count on Linux keeping GCC relevant forever. Doubly so with the inevitable rustc requirement.
I could be missing something, but I don't see any suggestion that you need a specific forked tree with patches to build with LLVM, and I've seen people filing bugs about using LLVM sanitizers to build the vanilla tree, so I don't think the expectation is that you need to apply a huge out of tree patchset for this to work any more?
AFAICT from the issues page, Clang and binutils/LLVM tools work fine with no patches for the mainstream archs and when not trying to be super-fancy with custom flags. The more non-mainstream one goes with arch or flags the more likely one will run into something.
I'm fairly confident the FSF would keep it alive for a long time, but I don't see why it would be necessarily be a priority for the Linux devs to keep GCC forever.
Azure Sphere OS is also GCC only, despite Microsoft's new foundled love for clang.