[llvm-pdbdump] Minor prepatory refactor of Class Def Dumper.

In a followup patch I intend to introduce an additional dumping
mode which dumps a graphical representation of a class's layout.
In preparation for this, the text-based layout printer needs to
be split out from the graphical layout printer, and both need
to be able to use the same code for printing the intro and outro
of a class's definition (e.g. base class list, etc).

This patch does so, and in the process introduces a skeleton
definition for the graphical printer, while currently making
the graphical printer just print nothing.

NFC

llvm-svn: 300134