Constant folder should use error_mark_node

[philberty]

The constant folder implementation should take advantage of error nodes.

• return error expression

  gccrs/gcc/rust/typecheck/rust-hir-const-fold.h

  Line 217 in 28f527c

  return nullptr;

  the default value of the folded result is already an error expression

  gccrs/gcc/rust/typecheck/rust-hir-const-fold.h

  Line 395 in 28f527c

  : ConstFoldBase (), folded (ctx->get_backend ()->error_expression ())

• update error handling code now that the result is at least error_mark_node this no longer needs a nullptr check

  gccrs/gcc/rust/typecheck/rust-hir-type-check.cc

  Lines 395 to 401 in 28f527c

  	auto capacity = ConstFold::ConstFoldExpr::fold (type.get_size_expr ());
  	if (capacity == nullptr)
  	return;
  	TyTy::BaseType *base = TypeCheckType::Resolve (type.get_element_type ());
  	translated = new TyTy::ArrayType (type.get_mappings ().get_hirid (), capacity,
  	TyTy::TyVar (base->get_ref ()));