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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@226751 91177308-0d34-0410-b5e6-96231b3b80d8