aco: Migrate RA to use std::optional

aco: Migrate RA to use std::optional

The use of std::optional simplifies expressions and would be useful for some
upcoming RA tweaks.

C++17 has been available since the merge of rusticl and should be safe to use as
far as packaging is concerned.

A few style choices are:
- Testing for emptiness uses implicit bool conversion.
- Constructing an empty value uses {}.
- Constructing a filled value uses the implicit conversion constructor.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20125>