Emit DeferredDeclsToEmit in a DFS order.

Currently we emit DeferredDeclsToEmit in reverse order. This patch changes that.

The advantages of the change are that

* The output order is a bit closer to the source order. The change to
test/CodeGenCXX/pod-member-memcpys.cpp is a good example.

* If we decide to deffer more, it will not cause as large changes in the
estcases as it would without this patch.

llvm-svn: 226751