Attach the dummy Parameter to PARAM_INFO

As opposed to relying on PRECEDING_DECL (which most of the
time is *not* the parameter we need) and DUMMY_PARAM
Make sure WHY returns an Any if there is none for Parameters
Get rid of DUMMY_PARAM
Have WHY assign WHEREFORE upon retrieval

It makes it easier to reason about when this is
getting set, and it's got few fewer special cases.