unique_ptr-ify FileSystemStatCache::setNextStatCache

And in the process, discover that FileManager::removeStatCache had a
double-delete when removing an element from the middle of the list (at
the beginning or the end of the list, there was no problem) and add a
unit test to exercise the code path (which successfully crashed when run
(with modifications to match the old API) without this patch applied)

llvm-svn: 215388