c++: missing -Wunused-value for !<expr> [PR114104]

c++: missing -Wunused-value for !<expr> [PR114104]

Here we're neglecting to issue a -Wunused-value warning for suitable !
operator expressions, and in turn for != operator expressions that are
rewritten as !(x == y), only because we don't call warn_if_unused_value
on TRUTH_NOT_EXPR since its class is tcc_expression.  This patch makes
us also consider warning for TRUTH_NOT_EXPR and also for ADDR_EXPR.

	PR c++/114104

gcc/cp/ChangeLog:

	* cvt.cc (convert_to_void): Call warn_if_unused_value for
	TRUTH_NOT_EXPR and ADDR_EXPR as well.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wunused-20.C: New test.

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