Change lookup_hir_implitem return type

Change lookup_hir_implitem return type

Wrap the return type within an optional. Now return the parent id within
a pair instead of taking an out reference.

gcc/rust/ChangeLog:

	* backend/rust-compile-item.cc (CompileItem::visit): Change call site
	to accept new return type.
	* backend/rust-compile-resolve-path.cc (HIRCompileBase::query_compile):
	Likewise.
	* backend/rust-mangle-v0.cc (v0_path): Likewise.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::visit):
	Likewise.
	* checks/lints/rust-lint-marklive.cc (MarkLive::go): Likewise.
	(MarkLive::visit): Likewise.
	* typecheck/rust-type-util.cc (query_type): Likewise.
	* util/rust-hir-map.cc (Mappings::insert_hir_implitem): Likewise.
	(Mappings::lookup_hir_implitem): Change return type.
	* util/rust-hir-map.h: Update the function's prototype.

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