internal/trace: correctly handle GoUndetermined for GoroutineSummary

internal/trace: correctly handle GoUndetermined for GoroutineSummary

Currently the trace summarization incorrectly handles GoUndetermined by
treating it too much like GoNotExist. In particular, it should be
accumulating all the time since the start of the trace in a particular
bucket, but it doesn't, so that instead gets counted as "unknown time"
because the "creation time" is at the start of the trace.

This change fixes the problem by simply doing the accumulation. It's
very straightforward. It also side-steps some other inaccuracies, like
associating a goroutine that is being named with the current task. I
don't think this can ever actually happen in practice, but splitting up
the two cases, GoUndetermined and GoNotExist, fixes it.

Fixes #76716.

Change-Id: I3ac1557044f99c92bada2cb0e124b2192b1d6ebb
Reviewed-on: https://go-review.googlesource.com/c/go/+/728822
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Nick Ripley <nick.ripley@datadoghq.com>