[ADT] Speculative attempt to fix build bot issues with r290952.

This just removes the usage of llvm::reverse and llvm::seq. That makes
it harder to handle the empty case correctly and so I've also added
a test there.

This is just a shot in the dark at what might be behind the buildbot
failures. I can't reproduce any issues locally including with ASan...
I feel like I'm missing something...

llvm-svn: 290954