[PDB] Fix an issue writing the publics stream.

In the refactor to merge the publics and globals stream, a bug
was introduced that wrote the wrong value for one of the fields
of the PublicsStreamHeader.  This caused debugging in WinDbg
to break.

We had no way of dumping any of these fields, so in addition to
fixing the bug I've added dumping support for them along with a
test that verifies the correct value is written.

llvm-svn: 310439