cmLocalGenerator: ignore scanned sources for unity builds

Source/cmLocalGenerator.cxx

@@ -3134,6 +3134,15 @@ void cmLocalGenerator::AddUnityBuild(cmGeneratorTarget* target)
     std::vector<cmSourceFile*> sources;
     target->GetSourceFiles(sources, configs[ci]);
     for (cmSourceFile* sf : sources) {
+      // Files which need C++ scanning cannot participate in unity builds as
+      // there is a single place in TUs that may perform module-dependency bits
+      // and a unity source cannot `#include` them in-order and represent a
+      // valid TU.
+      if (sf->GetLanguage() == "CXX"_s &&
+          target->NeedDyndepForSource("CXX", configs[ci], sf)) {
+        continue;
+      }
+
       auto mi = index.find(sf);
       if (mi == index.end()) {
         unitySources.emplace_back(sf);