[OpenACC] Implement `self` clause for compute constructs (#88760)

`self` clauses on compute constructs take an optional condition
expression. We again limit the implementation to ONLY compute const

[OpenACC] Implement `self` clause for compute constructs (#88760)

`self` clauses on compute constructs take an optional condition
expression. We again limit the implementation to ONLY compute constructs
to ensure we get all the rules correct for others. However, this one
will be particularly complicated, as it takes a `var-list` for `update`,
so when we get to that construct/clause combination, we need to do that
as well.

This patch also furthers uses of the `OpenACCClauses.def` as it became
useful while implementing this (as well as some other minor refactors as
I went through).

Finally, `self` and `if` clauses have an interaction with each other, if
an `if` clause evaluates to `true`, the `self` clause has no effect.
While this is intended and can be used 'meaningfully', we are warning on
this with a very granular warning, so that this edge case will be
noticed by newer users, but can be disabled trivially.

show more ...