[llvm-dwp] Use buffer_stream if output file is not seekable (e.g. "-")

Reviewers: dblaikie, pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51707

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341509 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed