[clang] clang-format: Sort includes by stem rather than full filename (PR #137840)

[Björn Schäpers via cfe-commits]

HazardyKnusperkeks wrote:

> > You definitely need some tests. And I'm not sure if everyone would like that, of if it should be an option.
> 
> I was hoping to get some feedback on whether it should be an option or not before adding tests.

I would be in favor.

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