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

[Owen Pan via cfe-commits]

================
@@ -4385,8 +4385,18 @@ struct FormatStyle {
     ///    #include "B/a.h"           #include "a/b.h"
     /// \endcode
     bool IgnoreCase;
+    /// When sorting includes in each block, only take file extensions into
+    /// account if two includes compare equal otherwise.
+    /// \code
+    ///    true:                          false:
+    ///    # include "A.h"     vs.        # include "A-util.h"
----------------
owenca wrote:

```suggestion
    ///    # include "A.h"         vs.    # include "A-util.h"
```
as suggested before.

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