[dev.regabi] cmd/compile: simplify dwarfgen.declPos

The previous code was way overcomplicating things. To find out if a
variable is a closure pseudo-variable, one only needs to check
IsClosureVar. Checking Captured and Byval are only meant to be used by
closure conversion.

Passes toolstash -cmp.

Change-Id: I22622cba36ba7f60b3275d17999a8b6bb7c6719a
Reviewed-on: https://go-review.googlesource.com/c/go/+/280995


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>