SI-7214 outer check based on dealiased pattern type.

A Typed Pattern (_: T) is more than `.isInstanceOf`: if `T` is a
path dependent type, the scrutinee's $outer reference is also
compared against the prefix of `T`.

The code that synthesises this is split into two places.
`needsOuterCheck` determines whether to add this check, based on
the type `T`, and the type of the scrutinee. If it gives the
go-ahead, `treeCondStrategy.outerCheck` synthesizes the check.

The new test case demonstrates the problems caused by the failure
to dealias in `needsOuterCheck`: it could either wrongly lead to
synthesis of an outer test (which would crash), or wrongly omit
the outer test (meaning overly liberal matching.)

A simple `dealias` remedies this. `dealiasWiden` is *not*
appropriate here; we need to keep hold of singleton types.

I'll also note that there is already a little slack between these
methods, as commented:

> ExplicitOuter replaces `Select(q, outerSym) OBJ_EQ expectedPrefix`
> by `Select(q, > outerAccessor(outerSym.owner)) OBJ_EQ expectedPrefix`
> if there's an outer accessor, otherwise the condition becomes `true`
> TODO: can we improve needsOuterTest so there's always an outerAccessor?

So this is probably a fragile area that warrants a careful review
with a view to design improvements.
Merge pull request #2199 from retronym/ticket/7214

SI-7214 outer check based on dealiased pattern type.