[clang] clang-format: Add IncludeSortKey option (PR #137840)

[Daan De Meyer via cfe-commits]

DaanDeMeyer wrote:

@owenca @HazardyKnusperkeks Rebased onto latest main to take advantage of @owenca's rework and addressed comments.

https://github.com/llvm/llvm-project/pull/137840