GCC 7.x doesn't like it (-Werror=subobject-linkage) when a class
either derives from a class or has a member field of a type that
was declared in an anonymous namespace.
It is also opposed (-Werror=attributes) to visibility attributes
being defined at explicit template instantiations.
GCC 8.x further has reservations (-Werror=class-memaccess) about
letting memset/memcpy modify areas within non-POD objects.

Change-Id: Ic5107bb5ee3af6233e3741e3ef78d03a0a84005a
Reviewed-on: https://chromium-review.googlesource.com/1208306
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56106}