[llvm-pdbdump] Introduce an abstraction for the output style.

This opens the door to introducing a YAML outputter which can be
used for machine consumption.  Currently the yaml output style
is unimplemented and returns an error if you try to use it.

Reviewed By: rnk, ruiu
Differential Revision: http://reviews.llvm.org/D20967

llvm-svn: 271712