This avoids the compile-time overhead of computing them twice. It also fixes
an issue where the hash computed after typeck is differen than the hash before,
because typeck mutates the def-map in place.
Fixes#35549.
Fixes#35593.
This checks the `previous_work_products` data from the dep-graph and
tries to simply copy a `.o` file if possible. We also add new
work-products into the dep-graph, and create edges to/from the dep-node
for a work-product.
