Reland rL341509: "[llvm-dwp] Use buffer_stream if output file is not seekable (e.g. "-")"

It caused ambiguity between llvm::cl::Optional and llvm::Optional, which
has been fixed by dropping `using namespace cl;` in favor of explicit
cl:: qualified names.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341586 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed