Improve the caching of debuginfo Objective C interface types.
Generate forward declarations that are RAUW'd by finalize().
We thus avoid outputting the same type several times in multiple
stages of completion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176820 91177308-0d34-0410-b5e6-96231b3b80d8