cmd/compile: use a non-fragile test for "does f contain closure c?"

cmd/compile: use a non-fragile test for "does f contain closure c?"

The old test relied on naming conventions.  The new test
uses an explicit parent pointer chain initialized when the
closures are created (in the same place that the names
used in the older fragile test were assigned).

Fixes #70035.

Change-Id: Ie834103c7096e4505faaff3bed1fc6e918a21211
Reviewed-on: https://go-review.googlesource.com/c/go/+/622656
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>