c++: more ahead-of-time -Wparentheses warnings

c++: more ahead-of-time -Wparentheses warnings

Now that we don't have to worry about looking through NON_DEPENDENT_EXPR,
we can easily extend the -Wparentheses warning in convert_for_assignment
to consider (non-dependent) templated assignment operator expressions as
well, like r14-4111-g6e92a6a2a72d3b did in maybe_convert_cond.

gcc/cp/ChangeLog:

	* cp-tree.h (maybe_warn_unparenthesized_assignment): Declare.
	* semantics.cc (is_assignment_op_expr_p): Generalize to return
	true for any assignment operator expression, not just one that
	has been resolved to an operator overload.
	(maybe_warn_unparenthesized_assignment): Factored out from ...
	(maybe_convert_cond): ... here.
	(finish_parenthesized_expr): Mention
	maybe_warn_unparenthesized_assignment.
	* typeck.cc (convert_for_assignment): Replace -Wparentheses
	warning logic with maybe_warn_unparenthesized_assignment.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wparentheses-13.C: Strengthen by expecting that
	we issue the -Wparentheses warnings ahead of time.
	* g++.dg/warn/Wparentheses-23.C: Likewise.
	* g++.dg/warn/Wparentheses-32.C: Remove xfails.