SI-7886 unsoundness in pattern matcher.

I tracked down what was behind the issue described here:
```
  // TODO: fix the illegal type bound in pos/t602 -- type inference
  // messes up before we get here:
  /*override def equals(x$1: Any): Boolean = ...
         // Span[Any] --> Any is not a legal type argument for Span!
         val o5: Option[com.mosol.sl.Span[Any]] =
  */
```
...which led straight to the unsoundness seen in neg/t7886.
It is dangerous to have an expected type of "Any" because
the type system will blithely ignore kind errors, since "Any"
can absorb anything. The consequence in this instance was
that inferring the case constructor for a type like

  ```Foo[T <: Bound]```

if done with expected type Any, this would come up with Foo[Any].
I altered it to use expected type Foo[T], which lets the dummy
type parameter survive to carry the bound forward and restores
sense to the inference. The before/after output for -Xprint:patmat
on pos/t602.scala is:
```diff
15c15
<         if (x1.isInstanceOf[com.mosol.sl.Span[Any]])
---
>         if (x1.isInstanceOf[com.mosol.sl.Span[K]])
17c17
<             <synthetic> val x2: com.mosol.sl.Span[Any] = \
  (x1.asInstanceOf[com.mosol.sl.Span[Any]]: com.mosol.sl.Span[Any]);
---
>             <synthetic> val x2: com.mosol.sl.Span[K] = \
  (x1.asInstanceOf[com.mosol.sl.Span[K]]: com.mosol.sl.Span[K]);
19,20c19,20
<               val low$0: Option[Any] = x2.low;
<               val high$0: Option[Any] = x2.high;
---
>               val low$0: Option[K] = x2.low;
>               val high$0: Option[K] = x2.high;
```
A file in the library depended (needlessly) on the unsoundness.
It was easy to fix but reminds us this may affect existing code.