This change adds a basic framework for doing baselined integration tests.
This is very similar to what we do elsewhere with generated files and
tests that read them from resources.
What's here now is the support to do this kind of baselining with IR in a
pretty readable serialization format.
This is a building block and the intent is that we'd do something similar
in the future for syntax nodes and C# source.
Looking at the code of the tests in particular, we'll also build the
ability to capture the documents at key points (such as before/after a
targeted phase) and then verify them in the same manner.
Ryan Nowak