gccrs: ast: Fix attribute tokenstreams

gccrs: ast: Fix attribute tokenstreams

Attributes were not converted to a correct tokenstream, furthermore meta
items containers and token trees attribute arguments were not even
implemented. This commit fix the literal attribute type and implement
both unimplemented types.

gcc/rust/ChangeLog:

	* ast/rust-ast-tokenstream.cc (TokenStream::visit): Fix existing
	and implement remaining attribute visitors.

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