c++: Fix bogus -Wparentheses warning [PR95344]

Since r267272, which added location wrappers, cp_fold loses
TREE_NO_WARNING on a MODIFY_EXPR that finish_parenthesized_expr set, and
that results in a bogus -Wparentheses warning.

I.e., previously we had "b = 1" but now we have "VIEW_CONVERT_EXPR<bool>(b) = 1"
and cp_fold_maybe_rvalue folds away the location wrapper and so we do
2718             x = fold_build2_loc (loc, code, TREE_TYPE (x), op0, op1);
in cp_fold and the flag is lost.

	PR c++/95344
	* cp-gimplify.c (cp_fold) <case MODIFY_EXPR>: Don't set
	TREE_THIS_VOLATILE here.
	(cp_fold): Set it here along with TREE_NO_WARNING.

	* c-c++-common/Wparentheses-2.c: New test.