Add a -Wclass-varargs to warn on objects of any class type being passed through an...

Add a -Wclass-varargs to warn on objects of any class type being passed through an ellipsis. Since C++11 relaxed the rules on this, we allow a lot more bad code through silently, such as:

  const char *format = "%s";
  std::experimental::string_view view = "foo";
  printf(format, view);

In this case, not only warn about a class type being used here, but also suggest that calling c_str() might be a good idea.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202461 91177308-0d34-0410-b5e6-96231b3b80d8