clang-format: Sort includes by stem rather than full filename

Sorting by stem gives nicer results when various header file names
are substrings of other header file names, for example, a CLI application
with a main header named analyze.h and a analyze-xxx.h header for each
subcommand currently will always put analyze.h last after all the
analyze-xxx.h headers, but putting analyze.h first instead of last is arguable
nicer to read.

TLDR; Instead of

"""
/#include "analyze-blame.h"
/#include "analyze.h"
"""

We'll now get

"""
/#include "analyze.h"
/#include "analyze-blame.h"
"""

Author: DaanDeMeyer

clang/lib/Format/Format.cpp

@@ -3219,17 +3219,19 @@ static void sortCppIncludes(const FormatStyle &Style,
 
   if (Style.SortIncludes == FormatStyle::SI_CaseInsensitive) {
     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);
+      const auto LHSStem = llvm::sys::path::stem(Includes[LHSI].Filename);
+      const auto RHSStem = llvm::sys::path::stem(Includes[RHSI].Filename);
+      const auto LHSStemLower = LHSStem.lower();
+      const auto RHSStemLower = RHSStem.lower();
+      return std::tie(Includes[LHSI].Priority, LHSStemLower, LHSStem) <
+             std::tie(Includes[RHSI].Priority, RHSStemLower, RHSStem);
     });
   } else {
     stable_sort(Indices, [&](unsigned LHSI, unsigned RHSI) {
-      return std::tie(Includes[LHSI].Priority, Includes[LHSI].Filename) <
-             std::tie(Includes[RHSI].Priority, Includes[RHSI].Filename);
+      const auto LHSStem = llvm::sys::path::stem(Includes[LHSI].Filename);
+      const auto RHSStem = llvm::sys::path::stem(Includes[RHSI].Filename);
+      return std::tie(Includes[LHSI].Priority, LHSStem) <
+             std::tie(Includes[RHSI].Priority, RHSStem);
     });
   }