gccrs: Change return type to optional in get_lang_item

gccrs: Change return type to optional in get_lang_item

Wrap the function's return type with an optional.

gcc/rust/ChangeLog:

	* typecheck/rust-autoderef.cc: Adapt calling code to new return type.
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit):
	Likewise.
	(TypeCheckExpr::resolve_operator_overload): Likewise.
	* typecheck/rust-tyty-bounds.cc (TypeBoundsProbe::assemble_builtin_candidate):
	Likewise.
	* typecheck/rust-tyty.cc (ClosureType::setup_fn_once_output): Likewise.
	* util/rust-hir-map.cc (Mappings::get_lang_item): Change return type.
	* util/rust-hir-map.h: Update the function's prototype.

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