Fix a warning with generating the attribute documentation; NFC.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223463 91177308-0d34-0410-b5e6-96231b3b80d8
Temporarily reverting r223443 due to bot breakage.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223465 91177308-0d34-0410-b5e6-96231b3b80d8