Follow-up to r214408: Warn on other callee-cleanup functions without prototype too.

According to lore, we used to verifier-fail on:

  void __thiscall f();
  int main() { f(1); }

So that's fixed now. System headers use prototype-less __stdcall functions,
so make that a warning that's DefaultError -- then it fires on regular code
but is suppressed in system headers.

Since it's used in system headers, we have codegen tests for this; massage
them slightly so that they still compile.