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When GHC attempts to unify a metavariable with a type containing
foralls, it will be rejected as an occurrence of impredicativity.
GHC was /not/ extending the same treatment to predicate types, such
as in the following (erroneous) example from #11514:

```haskell
foo :: forall a. (Show a => a -> a) -> ()
foo = undefined
```

This will attempt to instantiate `undefined` at
`(Show a => a -> a) -> ()`, which is impredicative. This patch
catches impredicativity arising from predicates in this fashion.

Since GHC is pickier about impredicative instantiations, some test
cases needed to be updated to be updated so as not to fall afoul of
the new validity check. (There were a surprising number of
impredicative uses of `undefined`!) Moreover, the `T14828` test case
now has slightly less informative types shown with `:print`. This is
due to a a much deeper issue with the GHCi debugger (see #14828).

Fixes #11514.

This patch is a pure refactoring, which I've wanted to do for
some time.  The main payload is

* Remove the wc_insol field from WantedConstraints;
  instead put all the insolubles in wc_simple

* Remove inert_insols from InertCans
  Instead put all the insolubles in inert_irreds

* Add a cc_insol flag to CIrredCan, to record that
  the constraint is definitely insoluble

Reasons

* Quite a bit of code gets slightly simpler
* Fewer concepts to keep separate
* Insolubles don't happen at all in production code that is
  just being recompiled, so previously there was a lot of
  moving-about of empty sets

A couple of error messages acutally improved.

I'd got the logic slightly wrong when reporting type errors
for insoluble 'given' equalities.  We suppress insoluble givens
under some circumstances (see Note [Given errors]), but we then
suppressed subsequent 'wanted' errors because the (suppressed)
'given' error "won".  Result: no errors at all :-(.

This patch fixes it and
 - Renames TcType.isTyVarUnderDatatype to the more
   perspicuous TcType.isInsolubleOccursCheck

In doing this I realise that I don't understand why we need
to keep the insolubles partitioned out separately at all...
but that is for another day.

Commit af21e388 fixed #12598. Let's add a test
to make sure it stays fixed.