re PR c++/85662 ("error: non-constant condition for static assertion" from __builtin_offsetof in C++)

	PR c++/85662
	* c-common.h (fold_offsetof_1): Removed.
	(fold_offsetof): Add TYPE argument defaulted to size_type_node and
	CTX argument defaulted to ERROR_MARK.
	* c-common.c (fold_offsetof_1): Renamed to ...
	(fold_offsetof): ... this.  Remove wrapper function.  Add TYPE
	argument, convert the pointer constant to TYPE and use size_binop
	with PLUS_EXPR instead of fold_build_pointer_plus if type is not
	a pointer type.  Adjust recursive calls.

	* c-fold.c (c_fully_fold_internal): Use fold_offsetof rather than
	fold_offsetof_1, pass TREE_TYPE (expr) as TYPE to it and drop the
	fold_convert_loc.
	* c-typeck.c (build_unary_op): Use fold_offsetof rather than
	fold_offsetof_1, pass argtype as TYPE to it and drop the
	fold_convert_loc.

	* cp-gimplify.c (cp_fold): Use fold_offsetof rather than
	fold_offsetof_1, pass TREE_TYPE (x) as TYPE to it and drop the
	fold_convert.

	* g++.dg/ext/offsetof2.C: New test.

From-SVN: r260119