cmd/compile: copy captured dictionary var to local var

When starting a closure that needs a dictionary, copy the closure
variable to a local variable. This lets child closures capture that
dictionary variable correctly.

This is a better fix for #47684, which does not cause problems
like #47723.

Fixes #47723
Update #47684

Change-Id: Ib5d9ffc68a5142e28daa7d0d75683e7a35508540
Reviewed-on: https://go-review.googlesource.com/c/go/+/343871


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