[LTO] Add error message on IO error in compileOptimizedToFile.

(No testcase because it's difficult to force an error here.)

Differential Revision: https://reviews.llvm.org/D26371

llvm-svn: 286177

llvm/lib/LTO/LTOCodeGenerator.cpp

@@ -257,6 +257,8 @@ bool LTOCodeGenerator::compileOptimizedToFile(const char **Name) {
   bool genResult = compileOptimized(&objFile.os());
   objFile.os().close();
   if (objFile.os().has_error()) {
+    Twine ErrMsg = "could not write object file: " + Filename.str();
+    emitError(ErrMsg.str());
     objFile.os().clear_error();
     sys::fs::remove(Twine(Filename));
     return false;