Add support for index resources (for a SlotIndex) to be relinquished.

When the SlotIndexes pass was introduced it was intended to support insertion
of code during register allocation. Removal of code was a minor consideration
(and raised the question of what to do about dangling SlotIndex objects pointing
to the erased index), so I opted to keep all indexes around indefinitely and
simply null out those that weren't being used.

Nowadays people are moving more code around (e.g. via HandleMove), which means
more zombie indexes. I want to start killing off indexes when we're done with
them to reclaim the resources they use up.
 

llvm-svn: 179834