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

compiler/rustc_trait_selection/src/traits/project.rs

@@ -889,7 +889,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
             debug!("recur cache");
             return Err(InProgress);
         }
-        Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
+        Err(ProjectionCacheEntry::NormalizedTy { ty, complete: _ }) => {
             // This is the hottest path in this function.
             //
             // If we find the value in the cache, then return it along