[AutoUpgrade] Don't upgrade intrinsics returning overloaded struct type

We only want to do the upgrade from named to anonymous struct
return if the intrinsic is declared to return a struct, but not
if it has an overloaded return type that just happens to be a
struct. In that case the struct type will be mangled into the
intrinsic name and there is no problem.

This should address the problem reported in
https://reviews.llvm.org/D122471#3416598.