Change lookup_node_to_hir return type to optional

Change lookup_node_to_hir return type to optional

Previous API was using a boolean and a pointer, this was not practical
and could be replaced with an optional.

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): Change call to use
	the returned optional.
	(CompileExpr::generate_closure_function): Likewise.
	* backend/rust-compile-resolve-path.cc (ResolvePathRef::resolve):
	Likewise.
	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): Likewise.
	* backend/rust-mangle-v0.cc (v0_path): Likewise.
	* checks/errors/privacy/rust-visibility-resolver.cc:
	Likewise.
	* checks/errors/rust-const-checker.cc (ConstChecker::visit): Likewise.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::visit):
	Likewise.
	* checks/lints/rust-lint-marklive.cc (MarkLive::visit): Likewise.
	(MarkLive::visit_path_segment): Likewise.
	* typecheck/rust-hir-trait-resolve.cc (TraitResolver::resolve_path_to_trait):
	Likewise.
	* typecheck/rust-hir-type-check-path.cc (TypeCheckExpr::resolve_root_path):
	Likewise.
	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::resolve_root_path):
	Likewise.
	(ResolveWhereClauseItem::visit): Likewise.
	* util/rust-hir-map.cc (Mappings::lookup_node_to_hir): Return an
	optional instead of a boolean.
	* util/rust-hir-map.h: Change function prototype to match the new
	return type.

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