Recovered panic values are wrapped and saved in Group.
Goexits are detected by a sentinel value set after the given function
returns normally. Wait propagates the first instance of a panic or
Goexit.

According to the runtime.Goexit after the code will not be executed,
with a bool, if f not call runtime.Goexit, is true,
determine whether to propagate runtime.Goexit.

Fixes golang/go#53757

Change-Id: Ic6426fc014fd1c4368ebaceef5b0d6163770a099
Reviewed-on: https://go-review.googlesource.com/c/sync/+/644575
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>