log/slog: don't panic on aliased Record

log/slog: don't panic on aliased Record

If the shared slice in a copied is modified, make a copy of it
and insert an attribute that warns of the bug.

Previously, we panicked, and panics in logging code should be avoided.

Change-Id: I24e9b0bf5c8cd09cf733e7dae8a82d025ef214e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/513760
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>