Make getTargetStreamer return a possibly null pointer.

This will allow it to be called from target independent parts of the main
streamer that don't know if there is a registered target streamer or not. This
in turn will allow targets to perform extra actions at specified points in the
interface: add extra flags for some labels, extra work during finalization, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199174 91177308-0d34-0410-b5e6-96231b3b80d8