internal/bisect: use f.Function instead of f.Func.Name in stack trace

internal/bisect: use f.Function instead of f.Func.Name in stack trace

For inlined frames f.Func is nil and f.Func.Name is an empty string.
f.Function is correct all the time.

Change-Id: I7c30f80e7176128ae5576b130d2891f884ee47c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/581996
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>