This is mostly described in:

https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1330711

...specifically note that your compiler now affects the output of your split config.  You can see that when I pick the above CL my split config changes:

https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1330712

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Do we need to do something like that CL?  I don't _think_ it's necessary for correctness unless we end up trying to enable an optional feature that's only enabled by clang.  

In theory when we do the normal kernel build it should realized that we changed compilers and cleanup it's config.

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I confirmed that at least with a no-op build my two CLs don't seem to have any real effect on build time.