c++: fix constexpr cast from void* diag issue [PR116741]

c++: fix constexpr cast from void* diag issue [PR116741]

The result of build_fold_indirect_ref can be a COMPONENT_REF in
which case using DECL_SOURCE_LOCATION will crash.  Look at its op1
instead.

	PR c++/116741

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression) <case CONVERT_EXPR>: If
	the result of build_fold_indirect_ref is a COMPONENT_REF, use its op1.
	Check DECL_P before calling inform.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/constexpr-voidptr4.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>