SparseSet: Add support for key-derived indexes and arbitrary key types.

This nicely handles the most common case of virtual register sets, but
also handles anticipated cases where we will map pointers to IDs.

The goal is not to develop a completely generic SparseSet
template. Instead we want to handle the expected uses within llvm
without any template antics in the client code. I'm adding a bit of
template nastiness here, and some assumption about expected usage in
order to make the client code very clean.

The expected common uses cases I'm designing for:
- integer keys that need to be reindexed, and may map to additional
  data
- densely numbered objects where we want pointer keys because no
  number->object map exists.

llvm-svn: 155227