x/tools: unified and longest builds are failing on the constraints package

[findleyr]

As of https://golang.org/cl/349709, which introduced the constraints package, the x/tools unified and longtest builders are failing, though in different ways.

Longtest builders are failing in go/ssa:
https://build.golang.org/log/5352d2134bdc9df4bef4a9e3dee22cc99d443e70
Perhaps it would suffice to skip standard library packages that use type parameters, for now.

Unified builders are failing in gcimporter: https://build.golang.org/log/5cdff1e5c507144f74953000b0c7335a32ea0587

The latter (unified) is a TryBot for x/tools, so should be fixed soon; I will take a look at it tomorrow.

CC @mdempsky @timothy-king @ianlancetaylor @griesemer

[cuonglm]

The CL also breaks longtest builders.

I sent https://go-review.googlesource.com/c/go/+/351853 to fix that.

[gopherbot]

Change https://golang.org/cl/352051 mentions this issue: go/ssa: skip the "constraints" package in TestStdlib

[gopherbot]

Change https://golang.org/cl/352050 mentions this issue: go/internal/gcimporter: temporarily skip the contraints package on the

[zpavlinovic]

Likely related #48525

[gopherbot]

Change https://golang.org/cl/352853 mentions this issue: go/internal/gcimporter: skip typeparam import tests on unified builders