createFunctionType requires only VFInfo and ScalarFTy

Not returning a pair anymore as the position can be queries directly from
VFInfo, which createFunctionType needs to have as an argument to begin
with.

Also getting the return type from the ScalarFTy, as the specification
does not encode in the mangled name such information. Therefore, the
VFInfo does not hold such info. If that changes, then it will make its
way into VFInfo and one could get it from there.