Significantly simplify CGExprAgg's logic about ignored results:

if we want to ignore a result, the Dest will be null.  Otherwise,
we must copy into it.  This means we need to ensure a slot when
loading from a volatile l-value.

With all that in place, fix a bug with chained assignments into
__block variables of aggregate type where we were losing insight into
the actual source of the value during the second assignment.

llvm-svn: 159630