gccrs: Use new constructors and control flow for operand

gccrs: Use new constructors and control flow for operand

gcc/rust/ChangeLog:

	* ast/rust-expr.h (struct InlineAsmOperand): changed to class
	(class InlineAsmOperand): Have appropriate constructor,
	and getter
	* expand/rust-macro-builtins-asm.cc (parse_reg_operand):
	Use the new implement constructors and new control flow.
	(parse_reg_operand_in): Likewise
	(parse_reg_operand_out): Likewise
	(parse_reg_operand_inout): Likewise
	(parse_reg_operand_const): Likewise