gccrs: Use error wrapper when required instead of nullptr

gccrs: Use error wrapper when required instead of nullptr

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Update function
	name.
	* ast/rust-ast-pointer-visitor.cc (PointerVisitor::visit): Likewise.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Likewise.
	* ast/rust-ast.cc (BreakExpr::as_string): Use getter function.
	(AttributeParser::parse_path_meta_item): Convert to expected type.
	(ReturnExpr::as_string): Change access to returned expr.
	* ast/rust-desugar-for-loops.cc (DesugarForLoops::DesugarCtx::make_break_arm):
	Likewise.
	* ast/rust-expr.h (class BreakExpr): Make expr in break optional.
	(class ReturnExpr): Make returned expr explicitely optional. Change the
	getters and introduce a  const getter.
	* expand/rust-cfg-strip.cc (CfgStrip::visit): Convert to expected type.
	* expand/rust-macro-builtins-asm.cc (parse_reg_operand_in): Update
	constructor call to new expected types.
	(parse_reg_operand_out): Likewise.
	(parse_reg_operand_inout): Likewise.
	(parse_llvm_operands): Likewise.
	* expand/rust-macro-builtins-format-args.cc (format_args_parse_expr):
	Likewise.
	(format_args_parse_arguments): Likewise.
	* expand/rust-macro-builtins-helpers.cc (try_expand_many_expr): Update
	value for tl::expected.
	* expand/rust-macro-builtins-include.cc (MacroBuiltin::include_handler):
	Likewise.
	* expand/rust-macro-expand.cc (transcribe_expression): Update
	constructor call with expected value.
	* hir/rust-ast-lower-expr.cc (ASTLoweringExpr::visit): Likewise.
	* parse/rust-parse-error.h (enum class): Add new error types for Expr
	and StructExprField.
	* parse/rust-parse-impl-expr.hxx: Explicitely handle return expr
	parsing failure. Update to fit new expected types.
	* parse/rust-parse-impl.hxx: Likewise.
	* resolve/rust-late-name-resolver-2.0.cc (Late::visit): Adapt to
	tl::expected.
	* ast/rust-desugar-while-let.cc (DesugarWhileLet::DesugarCtx::make_break_arm):
	Use optional for break expressions when missing instead of nullptr.
	* parse/rust-parse.h: Change function return type with expected. Remove
	error state from ExprOrStmt.

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