Add support for ambiguous use declarations

Add support for ambiguous use declarations

Glob use declarations may lead to ambiguous situation where two
definitions with the same name are imported in a given scope. The
compiler now handles shadowable items inserted after non shadowable
items. An error is now thrown when multiple shadowable items are imported
and used in the same rib.

gcc/rust/ChangeLog:

	* resolve/rust-early-name-resolver-2.0.cc (Early::visit): Adapt
	resolved type to the new API.
	(Early::visit_attributes): Retrieve the node id from the definition.
	* resolve/rust-forever-stack.h: Change the return type of getter
	functions. Those functions now return a definition type instead of a
	node id.
	* resolve/rust-forever-stack.hxx: Change member function implementation
	in the forever stack to accomodate it's API changes.
	* resolve/rust-late-name-resolver-2.0.cc (Late::visit): Use internal
	node id. Emit an error when resolving multiple ambiguous values.
	* resolve/rust-rib.cc (Rib::Definition::Definition): Add a default
	constructor.
	(Rib::Definition::is_ambiguous): Add a new function to determine
	whether a function definition is ambiguous or not.
	(Rib::Definition::to_string): Add a member function to convert a given
	definition to a string.
	(Rib::insert): Add new rules for value insertion in a rib. Insertion
	order does not impact the result anymore: inserting a shadowable value
	after a non shadowable one does not trigger an error anymore. All
	shadowable values inserted in a rib are kepts until being replaced by a
	non shadowable one.
	(Rib::get): Return a definition instead of a node id.
	* resolve/rust-rib.h: Update function prototypes.
	* resolve/rust-toplevel-name-resolver-2.0.cc (TopLevel::handle_use_glob):
	Update return value container to match the new function's prototype.
	(TopLevel::handle_use_dec): Likewise.
	(flatten_glob): Likewise.

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