Add a ValueMap<ValueOrSubclass*, T> type. ValueMap<Value*, T> is safe to use

even when keys get RAUWed and deleted during its lifetime. By default the keys
act like WeakVHs, but users can pass a third template parameter to configure
how updates work and whether to do anything beyond updating the map on each
action.

It's also possible to automatically acquire a lock around ValueMap updates
triggered by RAUWs and deletes, to support the ExecutionEngine.

llvm-svn: 84890