[libc++abi] Handle catch null pointer-to-object (#68076)

This addresses cases (currently failing) where we throw a null
pointer-to-object and fixes #64953.

We are trying to satisfy the following bullet from the C++ ABI 15.3:

* the handler is of type cv1 T* cv2 and E is a pointer type that can be
converted to the type of the handler by either or both of:

  - a standard pointer conversion (4.10 [conv.ptr]) not involving
    conversions to private or protected or ambiguous classes.

  - a qualification conversion.

The existing implementation assesses the ambiguity of bases by computing
the offsets to them; ambiguous cases are then when the same base appears
at different offsets. The computation of offset includes indirecting
through the vtables to find the offsets to virtual bases.

When the thrown pointer points to a real object, this is quite efficient
since, if the base is found, and it is not ambiguous and on a public
path, the offset is needed to return the adjusted pointer (and ...