weak: prevent unsafe conversions using weak pointers
Prevent conversions between Pointer types,
like we do for sync/atomic.Pointer.
Fixes #71583
Change-Id: I20e83106d8a27996f221e6cd9d52637b0442cea4
Reviewed-on: https://go-review.googlesource.com/c/go/+/647195
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>