Make some methods const. The only interesting change here is that

Make some methods const.  The only interesting change here is that
it changes raw_fd_ostream::preferred_buffer_size to return zero on
a scary stat failure instead of setting the stream to an error state.
This method really should not mutate the stream.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91740 91177308-0d34-0410-b5e6-96231b3b80d8