Reduce the PR20399 test case.

I couldn't get something /really/ obvious, and I imagine Richard Smith
might be able to provide some text explaining the sequence of steps
that's demonstrated by these files - but at least it's a bit simpler
now.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@218840 91177308-0d34-0410-b5e6-96231b3b80d8