cmd/compile: strengthen SetFields/Width safety guarantee

cmd/compile: strengthen SetFields/Width safety guarantee

It is currently possible in the compiler to create a struct type,
calculate the widths of types that depend on it,
and then alter the struct type.

transformclosure has local protection against this.
Protect against it at a deeper level.

This is preparation to call dowidth automatically,
rather than explicitly.

This is a re-roll of CL 38469.

Change-Id: Ic5b4baa250618504611fc57cbf51ab01d1eddf80
Reviewed-on: https://go-review.googlesource.com/38534
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>