go/types, types2: improve API for setting type parameter constraints in importers

[findleyr]

As we saw in #51219, the fact that SetConstraint eagerly computes the constraint type set makes the API awkward to use for importers.

We should investigate if this API restriction can be removed or improved.

CC @griesemer

[ianlancetaylor]

@findleyr What is the current status here? This issue is currently in the 1.19 milestone. Should it move to 1.20? To Backlog? Thanks.

[findleyr]

Let's bump to 1.20.

[gopherbot]

This issue is currently labeled as early-in-cycle for Go 1.23.
That time is now, so a friendly reminder to look at it again.

[griesemer]

@findleyr Anything to be done here at this point? Let's decide and then do or close.

[griesemer]

Bumping to 1.24 unless you want to close this, @findleyr .

[gopherbot]

This issue is currently labeled as early-in-cycle for Go 1.24.
That time is now, so a friendly reminder to look at it again.

[findleyr]

This is unlikely to get prioritized, since there are a finite number of importers and they are all working as intended. Therefore, this is an unfortunate API, but we probably won't improve it in the near future.

Keeping this open, but moving to the backlog.