Auto merge of #139756 - Zoxc:out-of-order-dep-graph, r=oli-obk

Allow out of order dep graph node encoding

This allows out of order dep graph node encoding by also encoding the index instead of using the file node order as the index.

`MemEncoder` is also brought back to life and used for encoding.

Both of these are done to enable thread-local encoding of dep graph nodes.

This is based on rust-lang/rust#139636.

Author: bors