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

[Owen Pan via cfe-commits]

================
@@ -3221,15 +3223,33 @@ static void sortCppIncludes(const FormatStyle &Style,
     stable_sort(Indices, [&](unsigned LHSI, unsigned RHSI) {
       const auto LHSFilenameLower = Includes[LHSI].Filename.lower();
       const auto RHSFilenameLower = Includes[RHSI].Filename.lower();
-      return std::tie(Includes[LHSI].Priority, LHSFilenameLower,
-                      Includes[LHSI].Filename) <
-             std::tie(Includes[RHSI].Priority, RHSFilenameLower,
-                      Includes[RHSI].Filename);
+      SmallString<128> LHSStem = Includes[LHSI].Filename;
+      SmallString<128> RHSStem = Includes[RHSI].Filename;
+      if (Style.IncludeStyle.IncludeSortKey ==
+          tooling::IncludeStyle::ISK_Stem) {
----------------
owenca wrote:

```suggestion
      SmallString<128> LHSStem, RHSStem;
      if (Style.IncludeStyle.IncludeSortKey ==
          tooling::IncludeStyle::ISK_Stem) {
        LHSStem = Includes[LHSI].Filename;
        RHSStem = Includes[RHSI].Filename;
```
So that they will be empty for `ISK_Path` for more efficient comparisons.

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