Batch mode driver remark

lib/Driver/Driver.cpp

@@ -716,6 +716,11 @@ Driver::buildCompilation(const ToolChain &TC,
   const bool ContinueBuildingAfterErrors =
       BatchMode || ArgList->hasArg(options::OPT_continue_building_after_errors);
 
+  // Issue a remark to facilitate recognizing the use of batch mode in the build
+  // log.
+  if (BatchMode)
+    Diags.diagnose(SourceLoc(), diag::remark_using_batch_mode);
+
   if (Diags.hadAnyError())
     return nullptr;