gccrs: Convert lookup return type to optional

gccrs: Convert lookup return type to optional

Remove usage of error state (but not error state itelf) and use an
optional to convey the missing value meaning instead.

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::generate_closure_fntype):
	Unwrap the optional.
	* backend/rust-compile.cc: Change return type container. Adapt code to
	new return type.
	* typecheck/rust-hir-dot-operator.cc: Likewise.
	* typecheck/rust-hir-path-probe.cc: Likewise.
	* typecheck/rust-hir-type-check-implitem.cc (TypeCheckImplItemWithTrait::visit):
	Likewise.
	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::visit):
	Likewise.
	* typecheck/rust-tyty-bounds.cc (TypeBoundPredicate::contains_item):
	Likewise.
	(TypeBoundPredicate::lookup_associated_item): Likewise.
	(TypeBoundPredicateItem::get_parent): Likewise.
	(TypeBoundPredicate::lookup_associated_type): Likewise.
	* typecheck/rust-tyty.cc (BaseType::satisfies_bound): Likewise.
	* typecheck/rust-hir-type-check-path.cc (TypeCheckExpr::visit): Change
	return type.
	* typecheck/rust-tyty.h: Likewise.

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