[Reproducers] Refactor reproducer info

In the original reproducer design, I expected providers to be more
dynamic than they turned out. For example, we don't have any instances
where one provider has multiple files. Additionally, I expected there to
be less locality between capture and replay, with the provider being
defined in one place and the replay code to live in another. Both
contributed to the design of the provider info.

This patch refactors the reproducer info to be something static. This
means less magic strings and better type checking. The new design still
allows for the capture and replay code to live in different places as
long as they both have access to the new statically defined info class.

I didn't completely get rid of the index, because it is useful for (1)
sanity checking and (2) knowing what files are used by the reproducer.

Differential revision: https://reviews.llvm.org/D56814

llvm-svn: 351501