c++: improve -Waddress warnings with *_cast [PR105569]

c++: improve -Waddress warnings with *_cast [PR105569]

This patch improves the diagnostic for -Waddress when it warns for

  if (dynamic_cast<A*>(&ref))
    // ...

where 'ref' is a reference, which cannot be null.  In particular, it
changes
warning: comparing the result of pointer addition '(((A*)ref) + ((sizetype)(*(long int*)((& ref)->B::_vptr.B + -24))))' and NULL
to
warning: the compiler can assume that the address of 'ref' will never be NULL

	PR c++/105569

gcc/cp/ChangeLog:

	* typeck.cc (warn_for_null_address): Improve the warning when
	the POINTER_PLUS_EXPR's base is of reference type.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Waddress-9.C: New test.