This patch implements a few of the optional suggestions from the
initial patch comitting libpdb. In particular, it implements a
virtual function out of line for each of the concrete classes.
A few other minor cleanups exist as well, such as using override
instead of virtual, etc.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228516 91177308-0d34-0410-b5e6-96231b3b80d8