one piece of code is responsible for the lifetime of every aggregate

slot.  The easiest way to do that was to bundle up the information
we care about for aggregate slots into a new structure which demands
that its creators at least consider the question.

I could probably be convinced that the ObjC 'needs GC' bit should
be rolled into this structure.
Implement generalized copy elision.  The main obstacle here is that
IR-generation must be much more careful about making sure that exactly

llvm-svn: 113962