Alternative fix for r184473.

This just seems a bit tidier/more principled. Based on a patch provided
by Adrian - with the only minor tweak that it needed to use
"getTypeOrNull" rather than "getCompletedTypeOrNull" since we don't
store declarations in the CompletedTypes cache.

No intended functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184509 91177308-0d34-0410-b5e6-96231b3b80d8