c++: Adjust location of is_*_base_of diagnostics

c++: Adjust location of is_*_base_of diagnostics

It makes more sense to point at the derived type (that didn't specify
the base class).

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_trait_expr): Use t2's location for
	is_base_of, is_pointer_interconvertible_base_of, and
	is_virtual_base_of.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_virtual_base_of_diagnostic2.C: Adjust
	diagnostic.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>