Adjust Win32 return value code transformation

[kennykerr]

Many trialing Win32 parameters include a retval attribute indicating that languages should turn these parameters into a logical return value if possible. The trouble is that this attribute is not broadly applied, so windows-bindgen attempts to infer it in more cases. This inference is necessarily conservative to avoid producing invalid or inefficient transformations. I did however notice that APIs that explicitly include the retval attribute were being skipped due to the conservative heuristic, so this update just ensures that explicit retval parameters are always transformed. This simplifies the new Windows shell sample #2789.