[llvm-dwarfdump] Simplify -o handling

[llvm-dwarfdump] Simplify -o handling

ToolOutputFile handles '-' so no need to specialize here.
Also, we neither reassign the variable nor pass it around, thus no need
to use std::unique_ptr<ToolOutputFile>.

exit(1) -> return 1;  to call the destructor of raw_fd_stream

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357051 91177308-0d34-0410-b5e6-96231b3b80d8