Enable -Wunused-private-field with -Wunused.

It found a lot of cruft in LLVM that I removed last week and I couldn't find any
false positives. Time to give it more testing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158317 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed