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.