Auto merge of #89831 - Aaron1011:project-caching-speedup, r=jackh726

Auto merge of #89831 - Aaron1011:project-caching-speedup, r=jackh726

Re-introduce concept of projection cache 'completion'

Instead of clearing out the cache entirely, we store
the intermediate evaluation result into the cache entry.
This accomplishes several things:

* We avoid the performance hit associated with re-evaluating
  the sub-obligations
* We avoid causing issues with incremental compilation, since
  the final evaluation result is always the same
* We avoid affecting other uses of the same `InferCtxt` which
  might care about 'side effects' from processing the sub-obligations
  (e,g. region constraints). Only code that is specifically aware
   of the new 'complete' code is affected