gccrs: Make TyTy::TupleType::get_unit_type cache its return value

This removes a usage of Resolver::get_unit_type_node_id in
rust-hir-type-check-expr.cc (the HIR::TupleExpr overload of
TypeCheckExpr::visit).

gcc/rust/ChangeLog:

	* typecheck/rust-tyty.cc
	(TupleType::get_unit_type): Remove parameter, cache return
	value.
	* typecheck/rust-tyty.h
	(TupleType::get_unit_type): Remove parameter.
	* resolve/rust-late-name-resolver-2.0.cc
	(Late::setup_builtin_types): Adjust calls to get_unit_type.
	* resolve/rust-name-resolver.cc
	(Resolver::generate_builtins): Likewise.
	* typecheck/rust-hir-type-check-expr.cc
	(TypeCheckExpr::visit): Likewise.
	* typecheck/rust-hir-type-check-implitem.cc
	(TypeCheckTopLevelExternItem::visit): Likewise.
	(TypeCheckImplItem::visit): Likewise.
	* typecheck/rust-hir-type-check-item.cc
	(TypeCheckItem::visit): Likewise.
	* typecheck/rust-hir-type-check-stmt.cc
	(TypeCheckStmt::visit): Likewise.
	* typecheck/rust-hir-type-check-type.cc
	(TypeCheckType::visit): Likewise.
	* typecheck/rust-hir-type-check.cc
	(TraitItemReference::get_type_from_fn): Likewise.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>
10 files changed