Adapt the ObjC stepping algorithm to deal with "selector-stubs" in clang.

Adapt the ObjC stepping algorithm to deal with "selector-stubs" in clang.

Clang is adding a feature to ObjC code generation, where instead of calling
objc_msgSend directly with an object & selector, it generates a stub that
gets passed only the object and the stub figures out the selector.

This patch adds support for following that dispatch method into the implementation
function.