issue a friendlier error if someone tries to send precompiled header to '-' (stdout)

this brings the error on
  clang -c foo.h -o -
closer to
  clang -c foo.h -o /dev/null

llvm-svn: 89948

clang/lib/Frontend/FrontendActions.cpp

@@ -222,7 +222,7 @@ void GeneratePTHAction::ExecuteAction() {
       CI.getFrontendOpts().OutputFile == "-") {
     // FIXME: Don't fail this way.
     // FIXME: Verify that we can actually seek in the given file.
-    llvm::errs() << "ERROR: PTH requires an seekable file for output!\n";
+    llvm::llvm_report_error("PTH requires an seekable file for output!");
     ::exit(1);
   }
   llvm::raw_fd_ostream *OS =