cmd/compile: fix missing descend in Addrtaken for closures.

ComputeAddrtaken needs to descend into closures, now that imported
bodies can include closures. The bug was that we weren't properly
setting Addrtaken for a variable inside a closure inside a function that
we were importing.

For now, still disable inlining of functions with closures for -G mode.
I'll enable it in a later change -- there are just a few fixes related
to the fact that we don't need to set Ntype for closure functions.

Added a test derived from the cilium repro in the issue.

Fixes #44370

Change-Id: Ida2a403636bf8740b471b3ad68b5474951811e19
Reviewed-on: https://go-review.googlesource.com/c/go/+/296649


Run-TryBot: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>