Demand: Clear distinction between Call SubDmd and eval Dmd (#21717)

In #21717 (closed) we saw a reportedly unsound strictness signature due to an unsound definition of plusSubDmd on Calls. This patch contains a description and the fix to the unsoundness as outlined in Note [Call SubDemand vs. evaluation Demand].

This fix means we also get rid of the special handling of -fpedantic-bottoms in eta-reduction. Thanks to less strict and actually sound strictness results, we will no longer eta-reduce the problematic cases in the first place, even without -fpedantic-bottoms.

So fixing the unsoundness also makes our eta-reduction code simpler with less hacks to explain. But there is another, more unfortunate side-effect: We unfix #21085 (closed), but fortunately we have a new fix ready: See Note [mkCall and plusSubDmd].

There's another change: I decided to make Note [SubDemand denotes at least one evaluation] a lot simpler by using plusSubDmd (instead of lubPlusSubDmd) even if both argument demands are lazy. That leads to less precise results, but in turn rids ourselves from the need for 4 different OpModes and the complication of Note [Manual specialisation of lub*Dmd/plus*Dmd]. The result is simpler code that is in line with the paper draft on Demand Analysis.

I left the abandoned idea in Note [Unrealised opportunity in plusDmd] for posterity. The fallout in terms of regressions is negligible, as the testsuite and NoFib shows.

        Program         Allocs    Instrs
--------------------------------------------------------------------------------
         hidden          +0.2%     -0.2%
         linear          -0.0%     -0.7%
--------------------------------------------------------------------------------
            Min          -0.0%     -0.7%
            Max          +0.2%     +0.0%
 Geometric Mean          +0.0%     -0.0%

Fixes #21717 (closed).