Fix a gcc warning.

In r181677 I removed this llvm_unreachable and it introduced a gcc
warning. Add it back.

Thanks to Patrik Hägglund for noticing it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181704 91177308-0d34-0410-b5e6-96231b3b80d8