gccrs: Make loop label truly optional

gccrs: Make loop label truly optional

A loop label error state was in use to represent missing loop label but
this may be easily forgotten and the optional nature of the label was
misrepresented.

gcc/rust/ChangeLog:

	* ast/rust-ast-builder.cc (Builder::block): Call with a nullopt instead
	of an error loop label.
	(WhileLetLoopExpr::as_string): Use getter function and adapt to
	newtype.
	* ast/rust-ast.cc (WhileLoopExpr::as_string): Likewise.
	(LoopExpr::as_string): Likewise.
	(BreakExpr::as_string): Likewise.
	(ForLoopExpr::as_string): Likewise.
	* ast/rust-expr.h (class BlockExpr): Make loop label optional.
	(class BreakExpr): Likewise.
	* expand/rust-derive-clone.cc (DeriveClone::clone_fn): Use nullopt.
	* expand/rust-derive-debug.cc (DeriveDebug::stub_debug_fn): Likewise.
	* expand/rust-derive-default.cc (DeriveDefault::default_fn): Likewise.
	* expand/rust-derive-eq.cc: Likewise.
	* parse/rust-parse-impl.h (Parser::parse_block_expr): Use optional
	for arguments.
	(Parser::parse_loop_expr): Likewise.
	(Parser::parse_while_loop_expr): Likewise.
	(Parser::parse_while_let_loop_expr): Likewise.
	(Parser::parse_for_loop_expr): Likewise.
	(Parser::parse_labelled_loop_expr): Likewise.
	(Parser::parse_loop_label): Return an optional.
	* parse/rust-parse.h: Update function prototype and use nullopt for
	default values.

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