Fix nasty bug in w/w for absence analysis

This dark corner was exposed by Trac #14285.  It involves the
interaction between absence analysis and INLINABLE pragmas.

There is a full explanation in Note [aBSENT_ERROR_ID] in MkCore,
which you can read there.  The changes in this patch are

* Make exprIsHNF return True for absentError, treating
  absentError like an honorary data constructor.

* Make absentError /not/ be diverging, unlike other error Ids.

This is all a bit horrible.

* While doing this I found that exprOkForSpeculation didn't
  have a case for value lambdas so I added one.  It's not
  really called on lifted types much, but it seems like the
  right thing