Now that the tree has been frozen for some time, update the tzdata
database to version 2018g (released 2018-10-26) for Go 1.12.

Updates #22487

Change-Id: I9e82bcdaef28d308643c08c9fd3472e4c14a196e
Reviewed-on: https://go-review.googlesource.com/c/151299
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>