Preparatory infrastructural work to support dynamically determining sizes of...

Preparatory infrastructural work to support dynamically determining sizes of ObjC types via the runtime

This is necessary because the byte size of an ObjC class type is not reliably statically knowable (e.g. because superclasses sit deep in frameworks that we have no debug info for)
The lack of reliable size info is a problem when trying to freeze-dry an ObjC instance (not the pointer, the pointee)

This commit lays the foundation for having language runtimes help in figuring out byte sizes, and having ClangASTType ask for runtime help
No feature change as no runtime actually implements the logic, and nowhere is an ExecutionContext passed in yet

llvm-svn: 227274