Reduce direct references to NodeDef in favor of Node and AttrSlice

This is one step towards replacing in-memory use of NodeDef with a customized
NodeInfo class.  There are still quite a few Node::def() references, but far fewer than before.  Those remaining require more work, either because they are part of kernel registration (which is a bunch of functions), copy and modify the NodeDef, etc.  Follow-on CLs will remove more.

RELNOTES: n/a
PiperOrigin-RevId: 156244933