Forcing it to be an error when there is no Documentation list specified for an attribute. This is a bit of a (harmless) hack, but the FIXME explains why and when this hack can be removed. It's a justified hack because this prevents attribute authors from forgetting to add documentation when they add a new attribute.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201524 91177308-0d34-0410-b5e6-96231b3b80d8