gccrs: ast: Use AnonConst for array type sizes

gccrs: ast: Use AnonConst for array type sizes

gcc/rust/ChangeLog:

	* ast/rust-expr.h: Add handling for deferred consts.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Likewise.
	* ast/rust-ast.cc (AnonConst::as_string): Likewise.
	(ArrayType::as_string): Likewise.
	* ast/rust-type.h (class ArrayType): Use AnonConst for sizes.
	* parse/rust-parse-impl.h (Parser::parse_anon_const): New function.
	(Parser::parse_slice_or_array_type): Call it.
	* parse/rust-parse.h: Declare it.