There are two cases where undefined-var-template warnings causes compilation failure on recent versions of Xcode. We should fix these as part of our toolchain upgrade effort.
This is a partial backport of Paul Hohensee's massive cleanup  of Xcode warnings. A complete backport of that chnageset is well beyond the scope of our toolchain upgrade work. But we do want to avoid having to disable any additional warnings as a result of this Xcode upgrade.