gccrs: Use new name resolver to compile constant items

gccrs: Use new name resolver to compile constant items

Constant items were handled only by the old resolver, this lead to an
ICE when using the new resolver on some rust code containing a constant
item as the new and the old resolver cannot be used at the same time.

gcc/rust/ChangeLog:

	* backend/rust-compile-item.cc (CompileItem::visit): Check the resolver
	flag and use the new one when required.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>