gccrs: Rework InlineAsmOperand

gccrs: Rework InlineAsmOperand

Not thrilled with some of this boilerplate, but it does seem like an
improvement.

gcc/rust/ChangeLog:

	* ast/rust-expr.h
	(InlineAsmOperand): Replace multiple mutually-exclusive tl::optional
	fields with a std::unique_ptr and modify nested classes to allow
	this. Also, make getters return references where possible.
	* expand/rust-macro-builtins-asm.cc
	(parse_reg_operand_out): Pass location when constructing
	InlineAsmOperand.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>