Move ObjCPropertyDecl to before ObjCContainerDecl.

After we add ObjCPropertyDecl::isClassProperty, we can use it in
ObjCContainerDecl to define filter to iterate over instance properties
and class properties.

This is the first patch in a series of patches to support class properties
in addition to instance properties in objective-c.

rdar://23891898

llvm-svn: 258727