gccrs: Change lookup_ast_item's return type

gccrs: Change lookup_ast_item's return type

Wrap the function's return type within an optional.

gcc/rust/ChangeLog:

	* metadata/rust-export-metadata.cc (ExportContext::emit_trait):
	Adapt call site to the new return type.
	(ExportContext::emit_function): Likewise.
	(ExportContext::emit_macro): Likewise.
	* util/rust-hir-map.cc (Mappings::lookup_ast_item): Change the
	function's return type.
	* util/rust-hir-map.h: Update the function's prototype.

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