Migrate stubbing transformation to be performed in the instance body

[celinval]

Proposed change:

Perform stubbing in the instance body instead of the generic one.

Motivation

This will simplify stubbing code, remove the redundant compilation and provide more precise error handling.

Background:

The stubbing transformation and validation are spread into multiple stages. The stubbing itself is done in the generic body by hijacking rustc's transformation query. Because of that, we have the following limitations:

1. If the function being stubbed is generic, we delay the instantiation requirements validation to be done during the reachability analysis.
2. We need to re-run the compilation for every set of stubs to invalidate the query cache and apply the right set of stubs.
3. We have to override collect_and_partition_mono_items query to avoid an ICE related to the delayed validation of the instantiation requirements.

There is a downside though. Today, we are able to apply the stubs to concrete playback, which wouldn't be possible until changes to the StableMIR are visible to the rust compiler.