[clang] sort additional module maps when serializing

[rmaz]

Summary

Sort additional module maps when serializing pcm files. This ensures
the MODULE_MAP_FILE record is deterministic across repeated builds.

[benlangmuir]

Good catch.

[rmaz]

rebase

[rmaz]

clang-format

clang/lib/Serialization/ASTWriter.cpp

AddPath(WritingModule->PresumedModuleMapFile.empty()
                 ? Map.getModuleMapFileForUniquing(WritingModule)->getName()
                 : StringRef(WritingModule->PresumedModuleMapFile),
             Record);
 
     // Additional module map files.
     if (auto *AdditionalModMaps =
             Map.getAdditionalModuleMapFiles(WritingModule)) {
       Record.push_back(AdditionalModMaps->size());
-      for (const FileEntry *F : *AdditionalModMaps)
+      SmallVector<const FileEntry *, 1> ModMaps(AdditionalModMaps->begin(), AdditionalModMaps->end());
+      llvm::sort(ModMaps, [](const FileEntry *A, const FileEntry *B) {
+        return A->getName() < B->getName();
+      });
+      for (const FileEntry *F : ModMaps)
         AddPath(F->getName(), Record);
     } else {
       Record.push_back(0);
     }
 
     Stream.EmitRecord(MODULE_MAP_FILE, Record);
   }