[dev.typeparams] cmd/compile: switch CaptureVars to use syntax.Walk

This CL refactors CaptureVars to use a visitor type so it's easier to
break out helper functions to review.

It also simplifies the quirks-mode handling of function literals:
instead of trying to maintain information about whether we're inside a
function literal or not, it now just rewrites the recorded position
information for any newly added free variables after walking the
function literal.

(Quirks mode is only for "toolstash -cmp"-style binary output testing
of normal code and will eventually be removed, so I don't think it's
important that this is an O(N^2) algorithm for deeply nested function
literals with lots of free variables.)

Change-Id: I0689984f6d88cf9937d4706d2d8de96415eaeee3
Reviewed-on: https://go-review.googlesource.com/c/go/+/330789


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>