This enables one to dump the various passpipelines in a yaml format. Other
pretty print formats can be added in the future as well if desired. Its intended
usage is to provide a source of pass pipeline information for external python
bug-reducing tools. By integrating this as a compiler-tool, we are guaranteed to
never have to update any of these tools in the face of passpipeline changes.
This commit adds the functionality, but does not change SILPassManager to use
it. The reason why I am doing this is so I can implement sil-opt pass bisecting
functionality in python using a tool that dumps the current pass pipelines
out. This will ensure that even in the face of changes to the pass pipelines,
everything should just work.
