reflect: add Value.CanConvert for 1.17 [freeze exception]

[ianlancetaylor]

The new method reflect.Value.CanConvert is a likely accept in #46746. If the acceptance becomes final as expected, I would like to request a freeze exception for https://golang.org/cl/334669 to add the method to the Go 1.17 release. Go 1.17 is the first release to permit a conversion that can panic dynamically (converting from a slice to a pointer-to-array if the slice is too short). Without reflect.Value.CanConvert, people using the reflect package must write awkward code to ensure that calls to reflect.Value.Convert do not panic even if reflect.Type.ConvertibleTo return true.

I believe this CL is safe for 1.17 even at this late stage as it only adds a new method. It does not change any existing code. It can't break any existing program.

CC @rsc @josharian @golang/release

[bcmills]

Marking as release-blocker because the decision itself needs to block the release.

(If we decide not to do this for 1.17, we can remove the release-blocker label.)

[toothrot]

We've discussed this as the release team, and approve the exception as it is unlikely to negatively impact the stability of the release, with the added benefit of being trivial to revert if the proposal is rejected in the next review meeting.

[ianlancetaylor]

Proposal accepted, CL submitted. Thanks.