[Reproducers] Improve reproducer API and add unit tests.

When I landed the initial reproducer framework I knew there were some
things that needed improvement. Rather than bundling it with a patch
that adds more functionality I split it off into this patch. I also
think the API is stable enough to add unit testing, which is included in
this patch as well.

Other improvements include:

 - Refactor how we initialize the loader and generator.
 - Improve naming consistency: capture and replay seems the least ambiguous.
 - Index providers by name and make sure there's only one of each.
 - Add convenience methods for creating and accessing providers.

Differential revision: https://reviews.llvm.org/D54616

llvm-svn: 347716