They get rewritten to NEWs, and they must be marked as escaping
so walk doesn't try to allocate them back onto the stack.

Fixes #15733

Change-Id: I433033e737c3de51a9e83a5a273168dbc9110b74
Reviewed-on: https://go-review.googlesource.com/23223
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>