[analyzer] Handle zeroing constructors for fields of structs with empty bases.

RegionStore tries to protect against accidentally initializing the same
region twice, but it doesn't take subregions into account very well. If
the outer region being initialized is a struct with an empty base class,
the offset of the first field in the struct will be 0. When we initialize
the base class, we may invalidate the contents of the struct by providing
a default value of Unknown (or some new symbol). We then go to initialize
the member with a zeroing constructor, only to find that the region at
that offset in the struct already has a value. The best we can do here is
to invalidate that value and continue; neither the old default value nor
the new 0 is correct for the entire struct after the member constructor call.

The correct solution for this is to track region extents in the store.

<rdar://problem/14914316>