Collect checks to run in CrossVersionChecks

[som-snytt]

Check for deprecation when inlining or constant folding, but avoid cyclic errors by deferring the exclusion check, which walks the owner chain, until CrossVersionChecks, where the check normally happens.

Fixes #2968

[odersky]

Is it reallly #2968 that is fixed here?

[odersky]

I'd like to understand the problem better before passing judgment.

At first glance, this looks like a lot of complications for a relatively small gain, so overall I am pretty down on this.

[odersky]

Adding something to the store is an expensive operation. It charges every time we copy a store with two additional slots to copy. if we look up the property not very often it's better to use a Property.

[som-snytt]

Scala 2 has a similar but simpler mechanism just to run functions after typer, with the same goal of avoiding cycles.

This could just be tacked onto CrossVersionChecks, more tightly coupled but also sufficient.

[odersky]

Yes, why don't we do this in cross version checks? That's where the other tests are done, right?

[som-snytt]

Constant folded ops currently disappear. Instead of trees preserved and constant types assigned.

Edit: also inlining.

[odersky]

What is an op that is constant folded and deprecated? Is there anything that's realistic in practice?

[odersky]

The other possibility is to order phases differently. We can move CrossVersionChecks earlier (with a bit of massaging) or drop trees of constant types later (currently done in FirstTransform).

[som-snytt]

The int-shift-long is deprecated; and the trick is just to avoid warning if an enclosing element is deprecated. Inlining opens the door to anything might be deprecated. (I also managed to trigger deprecation at the "inlined body retainer" member. There is a test that would normally trigger on experimental that way.)

This use case is not urgent, so I'll take some time to explore options.

(The ability to defer arbitrary checks is useful in Scala 2 to break out of the "ball of mud", but probably less useful here. I see Scala 2 has a DeferredRefCheck tree.)