optional pointer to consumer replaces NullDiagnosticConsumer placeholder

lib/FrontendTool/FrontendTool.cpp

@@ -1527,6 +1527,19 @@ createDispatchingDiagnosticConsumerIfNeeded(
       subConsumers.emplace_back(input.file(), std::move(subConsumer));
     return false;
   });
+  // For batch mode, the compiler must swallow diagnostics pertaining to
+  // non-primary files in order to avoid Xcode showing the same diagnostic
+  // multiple times. So, create a diagnostic "eater" for those non-primary
+  // files.
+  // To avoid introducing bugs into WMO or single-file modes, test for multiple
+  // primaries.
+  if (inputsAndOutputs.hasMultiplePrimaryInputs()) {
+    inputsAndOutputs.forEachInputNotProducingSupplementaryOutput(
+        [&](const InputFile &input) -> bool {
+          subConsumers.emplace_back(input.file(), nullptr);
+          return false;
+        });
+  }
 
   if (subConsumers.empty())
     return nullptr;