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

[Daan De Meyer via cfe-commits]

DaanDeMeyer wrote:

@owenca Thanks for the detailed review. Changed the name to IncludeSortIgnoreExtension, let me know if you have a better idea for the name.

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