Silence msvc warning on isl. NFC.

The warning is:

    isl_union_map.c(2041): warning C4221: nonstandard extension used: 'filter_user': cannot be initialized using address of automatic variable 'data'

for the following code (and others)

	struct isl_un_op_drop_user_data data = { &isl_set_is_wrapping };
	struct isl_un_op_control control = {
		.filter = &un_op_filter_drop_user,
		.filter_user = &data,
		.fn_map = &isl_set_wrapped_domain_map,
	};

llvm-svn: 329328