[analyzer] Add a new abstraction over all types of calls: CallEvent
This is intended to replace CallOrObjCMessage, and is eventually intended to be
used for anything that cares more about /what/ is being called than /how/ it's
being called. For example, inlining destructors should be the same as inlining
blocks, and checking attribute((nonnull)) should apply to the allocator
calls generated by operator new.