re PR c++/36921 (warning "comparison does not have mathematical meaning" is not correct for overloaded operators that do not return boolean)

	PR c++/36921
	* c-common.c (warn_about_parentheses): Remove ARG_UNUSED from
	arg_left.  Don't warn about X<=Y<=Z if comparison's type isn't
	integral.

	* g++.dg/warn/pr36921.C: New.

Co-Authored-By: Manuel López-Ibáñez <manu@gcc.gnu.org>

From-SVN: r142849