Allow a target to create a null streamer.

Allow a target to create a null streamer.

Targets can assume that a target streamer is present, so they have to be able
to construct a null streamer in order to set the target streamer in it to.

Fixes a crash when using the null streamer with arm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211358 91177308-0d34-0410-b5e6-96231b3b80d8