gccrs: Change trait getter to return references

gccrs: Change trait getter to return references

Having copy and any other constructor stuff might lead to a breakage in
the future where the node id differs due to a newly constructed
SimplePath node. This change will allow us to assert the NodeId is from
the ast and not any copy made in between.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (Attribute::get_traits_to_derive): Change
	return type to a vector of references.
	* ast/rust-ast.h: Update constructor.
	* expand/rust-expand-visitor.cc (ExpandVisitor::expand_inner_stmts):
	Update function call.

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