and OwningPtr instead of constructing only after all of sema is done.  This
has a couple of effects:
1. it fixes memory leaks from all the error cases in sema
2. it simplifies the code significantly.

The cost of this is that the error case now new's and delete's an expr where
it did not before, but we don't care about the perf of the error case.

llvm-svn: 45380