[analyzer] Invalidate union regions properly. Don't hesitate to load later.

We weren't invalidating our unions correctly. The previous behavior in
invalidateRegionsWorker::VisitCluster() was to direct-bind an UnknownVal
to the union (at offset 0).

For that reason we were never actually loading default bindings from our unions,
because there never was any default binding to load, and the value
that is presumed when there's no default binding to load
is usually completely incorrect (eg. UndefinedVal for stack unions).

The new behavior is to default-bind a conjured symbol (of irrelevant type)
to the union that's being invalidated, similarly to what we do for structures
and classes. Then it becomes safe to load the value properly.

Differential Revision: https://reviews.llvm.org/D45241

llvm-svn: 331563