[dev.typeparams] cmd/compile: simplify writer.collectDecls

The previous code for walking the syntax AST to find declarations
needed to know whether a declaration appeared within block scope, but
syntax.Crawl (née syntax.Walk) made that somewhat awkward.

This CL simplifies it a little, taking advantage of syntax.Walk's
support for keeping per-subtree state.

Change-Id: I03c7da8c44bec40f88e983852dc6bbab7e6ac13c
Reviewed-on: https://go-review.googlesource.com/c/go/+/330549


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