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

[Owen Pan via cfe-commits]

owenca wrote:

I would expect that file extensions be sorted regardless. For example:
```
#include "a.h"
#include "a.inc"
#include "a-util.def"
#include "a-util.h"
```

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