runtime: mark map bucket slots as empty during map clear

runtime: mark map bucket slots as empty during map clear

So iterators that are in progress can know entries have been deleted and
terminate the iterator properly.

Update #55002
Update #56351
Fixes #59411

Change-Id: I924f16a00fe4ed6564f730a677348a6011d3fb67
Reviewed-on: https://go-review.googlesource.com/c/go/+/481935
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>