Remove concept of 'completion' from the projection cache
Fixes#88910
When we initially store a `NormalizedTy` in the projection cache,
we discard all obligations that we can (while ensuring that we
don't cause any issues with incremental compilation).
Marking a projection cache entry as 'completed' discards all
obligations associated with it. This can only cause problems,
since any obligations stored in the cache are there for a reason
(e.g. they evaluate to `EvaluatedToOkModuloRegions`).
This commit removes `complete` and `complete_normalized` entirely.
e0c38af27c