Add a non-exiting diagnostic handler for LTO.
This is in order to give LTO clients a chance to do some clean-up before
terminating the process.

llvm-svn: 247461
diff --git a/llvm/tools/llvm-lto/llvm-lto.cpp b/llvm/tools/llvm-lto/llvm-lto.cpp
index ddde231..cdb1bca 100644
--- a/llvm/tools/llvm-lto/llvm-lto.cpp
+++ b/llvm/tools/llvm-lto/llvm-lto.cpp
@@ -216,8 +216,11 @@
     if (SetMergedModule && i == BaseArg) {
       // Transfer ownership to the code generator.
       CodeGen.setModule(std::move(Module));
-    } else if (!CodeGen.addModule(Module.get()))
+    } else if (!CodeGen.addModule(Module.get())) {
+      // Print a message here so that we know addModule() did not abort.
+      errs() << argv[0] << ": error adding file '" << InputFilenames[i] << "'\n";
       return 1;
+    }
   }
 
   // Add all the exported symbols to the table of symbols to preserve.