Define object, subobject, and non-divisible sub-subobject

[llvm-beanz]

In the language specification under Decl.Init.Agg, and in #358, we talk about objects and flattened orderings.

We should really have unified definitions earlier in the spec for "object", "subobject", "flattened ordering" and a notion of a non-divisible subobject (a subobject that does not contain subobjects).