Renames TyTrait to TyDynamic.

This moves closures to the (DefId, Substs) scheme like all other items,
and saves a word from the size of TyS now that Substs is 2 words.

This is done by creating a little space on the stack. Hokey, but it's
the simplest fix I can see.

Fix #36278. Fix #36294.

Fix some typeck bugs blocking drop tests

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.

The data tracked here was meant to compare the output of the
translation item collector to the set of translation items found
by the on-demand translator.

See issue #34151 for more information.