Template the sparse propagation solver instead of using void pointers

Summary:
This avoids using void * as the type of the lattice value and ugly casts needed to make that happen.
(If folks want to use references, etc, they can use a reference_wrapper).

Reviewers: davide, mssimpso

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D38476

llvm-svn: 314734