Since this really only makes sense for these two, change hte instance variable
to reflect whether we are writing a bytecode file or not. This makes it
reasonable to add bcwriter specific stuff to it as necessary.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10837 91177308-0d34-0410-b5e6-96231b3b80d8