This change adds rudimentary delete-before-create support (see
pulumi/pulumi#450). This cannot possibly be complete until we also
implement pulumi/pulumi#624, becuase we may try to delete a resource
while it still has dependent resources (which almost certainly will
fail). But until then, we can use this to manually unwedge ourselves
for leaf-node resources that do not support old and new resources
living side-by-side.
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