Change crate_num_to_nodeid return type

Change crate_num_to_nodeid return type

Change the function's return type to an optional.

gcc/rust/ChangeLog:

	* resolve/rust-ast-resolve-toplevel.h: Adapt the code to the new
	return type.
	* rust-session-manager.cc (Session::load_extern_crate): Likewise.
	* util/rust-hir-map.cc (Mappings::crate_num_to_nodeid): Change the
	return type.
	* util/rust-hir-map.h: Update the function's prototype.

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