cmd/compile: fix import of functions of multiple nested closure

cmd/compile: fix import of functions of multiple nested closure

For import of functions with closures, the connections among closure
variables are constructed on-the-fly via CaptureName(). For multiple
nested closures, we need to temporarily set r.curfn to each closure we
construct, so that the processing of closure variables will be correct
for any nested closure inside that closure.

Fixes #44335

Change-Id: I34f99e2822250542528ff6b2232bf36756140868
Reviewed-on: https://go-review.googlesource.com/c/go/+/294212
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>