[clang] Support variable empty lines between include categories (PR #183960)

[via cfe-commits]

owenca wrote:

> I consider myself relatively open to new formatting options, but for this one I don't know if that's really the right thing. Officially we have: https://clang.llvm.org/docs/ClangFormatStyleOptions.html#adding-additional-style-options
> 
> What's your opinion @owenca @mydeveloperday ?

I'm very hesitant to accept it. Let's see what @mydeveloperday has to say.

https://github.com/llvm/llvm-project/pull/183960