DebugInfoPDB: Make the symbol base case hold an IPDBSession ref.

Dumping a symbol often requires access to data that isn't inside
the symbol hierarchy, but which is only accessible through the
top-level session.  This patch is a pure interface change to give
symbols a reference to the session.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228542 91177308-0d34-0410-b5e6-96231b3b80d8