[Win64] Don't widen integer literal zero arguments to unprototyped function calls

The special case to widen the integer literal zero when passed to
variadic function calls should only apply to variadic functions, not
unprototyped functions. This is consistent with what MSVC does. In this
test case, MSVC uses a 4-byte store to pass the 5th argument to 'kr' and
an 8-byte store to pass the zero to 'v':

  void v(int, ...);
  void kr();
  void f(void) {
    v(1, 2, 3, 4, 0);
    kr(1, 2, 3, 4, 0);
  }

Aaron Ballman discovered this issue in https://reviews.llvm.org/D28166

llvm-svn: 290906