[ExecutionEngine] Change NotifyObjectEmitted/NotifyObjectFreed API.

This patch renames both methods (NotifyObjectEmitted -> notifyObjectLoaded, and
NotifyObjectFreed -> notifyObjectFreed), adds an abstract "ObjectKey" (uint64_t)
parameter to notifyObjectLoaded, and replaces the ObjectFile parameter for
notifyObjectFreed with an ObjectKey. Using an ObjectKey to track identify
events, rather than a reference to the ObjectFile, allows us to free the
ObjectFile after notifyObjectLoaded is called, saving memory.

https://reviews.llvm.org/D53773

llvm-svn: 348223