Move property API's up to ObjCContainerDecl (removing a lot of duplicate code).

Add isa/cast/dyncast support for ObjCContainerDecl.
Renamed classprop_iterator/begin/end to prop_iterator/begin/end (the class prefix was confusing).
More simplifications to Sema::ActOnAtEnd()...
Added/changed some FIXME's as a result of the above work.

llvm-svn: 61988