Ensure that we use the correct python to run the python based tools.
This also allows these tools to run on Windows which will not
necessarily associate the python script with an interpreter (python).
Recent changes that eliminated the -sil-serialize-all mode and adding this check to IRGen allow us to get rid of ExternalFunctionDefinitionsElimination and ExternalDefsToDecls passes, which are not needed anymore.
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.
