Emit -Waddress warnings for comparing address of reference against NULL

Emit -Waddress warnings for comparing address of reference against NULL

gcc/c-family/ChangeLog:

	PR c++/65168
	* c-common.c (c_common_truthvalue_conversion): Warn when
	converting an address of a reference to a truth value.

gcc/cp/ChangeLog:

	PR c++/65168
	* typeck.c (cp_build_binary_op): Warn when comparing an address
	of a reference against NULL.

gcc/testsuite/ChangeLog:

	PR c++/65168
	g++.dg/warn/Walways-true-3.C: New test.

From-SVN: r224455