reflect,runtime: add Value.Clear

reflect,runtime: add Value.Clear

Fixes #55002

Change-Id: I7d0f14cc54f67f2769b51d2efafc4ae3714f0e3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/457895
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>