reflect: fix Value.SetIterXXX to check for the read-only bit
v.SetIterXXX(i) is semantically identical to v.Set(i.XXX()).
If the latter panics for unexported values, so should the former.
This change may breaking some programs, but the change is justified
under the "Go 1 and the Future of Go Programs" document because
the "library has a bug that violates the specification".
In this case, the "reflect" package does not accurately match
the behavior of the Go language specification.
Also, this API was recently released, so the number of users
who could be depending on this behavior is hopefully lower.
Fixes #54628
Change-Id: If86ede51f286e38093f6697944c089f616525115
Reviewed-on: https://go-review.googlesource.com/c/go/+/425184
Auto-Submit: Joseph Tsai <joetsai@digital-static.net> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net> Reviewed-by: David Chase <drchase@google.com>