[ADT] Add a workaround for GCC miscompiling the trivially copyable Optional

I've seen random crashes with GCC 4.8, GCC 6.3 and GCC 7.3, triggered by
my Optional change. All of them affect a different set of targets. This
change fixes the instance of the problem I'm seeing on my local machine,
let's hope it's good enough for the other instances too.

llvm-svn: 322859