[lld/pdb] Create an empty public symbol record stream.

This is part of the continuing effort to increase parity between
LLD and MSVC PDBs.  link still doesn't like our PDBs, so the most
obvious thing to check was whether adding an empty publics stream
would get it to do something else.  It still fails in the same way
but at least this removes one more variable from the equation.
The next logical step would be to try creating an empty globals
stream.

Differential Revision: https://reviews.llvm.org/D35224

llvm-svn: 307598