[defns.dynamic.type] Say "most derived object" in the example

[frederick-vs-ja]

and

• don't say the static type of a pointer, because the difference between static and dynamic types is only present for glvalues of class types;
• say that the B* pointer is pointing to a B base class subobject, which avoids involving problematic cases of reinterpret_cast or static_cast from cv void* (see [expr.static.cast]/14).

Fixes #5762.
Fixes cplusplus/CWG#257.

[tkoeppe]

@jensmaurer: Could you have a look?

[jensmaurer]

This looks good.