[analyzer] Add more checks to the ObjC Ivar Invalidation checker.

Restructured the checker so that it could easily find two new classes of
issues:
 - when a class contains an invalidatable ivar, but no declaration of an
invalidation method
 - when a class contains an invalidatable ivar, but no definition of an
invalidation method in the @implementation.

The second case might trigger some false positives, for example, when
the method is defined in a category.