Change lookup_local_defid return type to optional

Change lookup_local_defid return type to optional

Make the API more convenient by changing the function's return type. We
can now differentiate between a stored null pointer and a missing value.

gcc/rust/ChangeLog:

	* util/rust-hir-map.cc (Mappings::insert_defid_mapping): Adapt call
	to new return type.
	(Mappings::insert_local_defid_mapping): Likewise.
	(Mappings::lookup_local_defid): Change return type to wrap it with an
	optional.
	* util/rust-hir-map.h: Update the function's prototype.

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