Fix incorrect FIXME.

The case that we wanted to write a test for cannot happen, as the
UnwrappedLineParser already protects against it. Added an assert to
prevent regressions of that assumption.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171720 91177308-0d34-0410-b5e6-96231b3b80d8