Downgrade error about stdcall decls with no prototype to a warning

Fixes PR21027.  The MIDL compiler produces code that does this.

If we wanted to improve the warning, I think we could do this:
  void __stdcall f(); // Don't warn without -Wstrict-prototypes.
  void g() {
    f(); // Might warn, the user probably meant for f to take no args.
    f(1, 2, 3); // Warn, we have no idea what args f takes.
    f(1); // Error, this is insane, one of these calls is broken.
  }

Reviewers: thakis

Differential Revision: http://reviews.llvm.org/D5481