cmd/compile: fix lexical block of captured variables

cmd/compile: fix lexical block of captured variables

Variables captured by a closure were always assigned to the root scope
in their declaration function. Using decl.Name.Defn.Pos will result in
the correct scope for both the declaration function and the capturing
function.

Fixes #21515

Change-Id: I3960aface3c4fc97e15b36191a74a7bed5b5ebc1
Reviewed-on: https://go-review.googlesource.com/56830
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>