this git branch takes 2 seconds to download, and shows you what patches are required (not only for the kernel, but also Clang and LLVM). it also provides a README that explains how to use llvm-setup for building an Clang/LLVM-compiled kernel....as for patches required;
51 patches for x86_64 (itself)
7 patches for LLVM
2 patches for Clang
plus other scripts / tools such as wrappers to allow clang/LLVM to substitute for GCC (not unlike the initial patches from LinuxDNA for Intel's C compiler, ICC - from a few years ago).... Now, i would imagine the second you tried to compile a more custom kernel, ie: patched for other things ...or started enabling features that have been disabled in their 'test' configs ~ you would probably run into a lot of other problems.