Allow mutation of moved upvars when they're marked as mutable.

[luqmana]

Fixes #11958.

[nikomatsakis]

/me will review

[nikomatsakis]

This looks good. Sorry it took me some time. The main thing that's missing are more tests. There ought to be tests for:

1. Mutation is allowed if declared mutable (you have this).
2. Mutation is NOT allowed if NOT declared mutable (didn't see this).
3. Tests for lint:
   • set the lint mode to forbid and add that to test Thread a session or semantic context through IL #1 so as to test that we don't get spurious errors here
   • set the lint mode to forbid and then write a test that reads let mut (with a closure) but does not mutate it, and check that you do in fact get a warning