Change return type for lookup_hir_item to optional

Change return type for lookup_hir_item to optional

gcc/rust/ChangeLog:

	* backend/rust-compile-resolve-path.cc (HIRCompileBase::query_compile):
	Adapt function call to new return type.
	* backend/rust-mangle-v0.cc (v0_path): Likewise.
	* checks/errors/rust-const-checker.cc (ConstChecker::check_function_call):
	Likewise.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::check_use_of_static):
	Likewise.
	(UnsafeChecker::check_function_call): Likewise.
	(UnsafeChecker::check_function_attr): Likewise.
	* checks/lints/rust-lint-marklive.cc (MarkLive::go): Likewise.
	(MarkLive::visit): Likewise.
	* typecheck/rust-hir-trait-resolve.cc (TraitResolver::resolve_path_to_trait):
	Likewise.
	* typecheck/rust-type-util.cc (query_type): Likewise.
	* util/rust-hir-map.cc (Mappings::insert_hir_item): Likewise.
	(Mappings::lookup_hir_item): Change function return type to use
	optional.
	* util/rust-hir-map.h: Update function prototype.

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