Extend type/borrow-check to guarantee values are returned

[nikomatsakis]

Functions can return without initializing their return place, which should be a type/borrow check error.

Currently the check for return place initialization was removed to enable the pending_outlives refactoring (threading outlives constraints as parameters instead of mutating TypeckEnv).

The test test_uninitialised_return_type demonstrates a function that returns without initializing the return place - this should fail but currently passes.

This validation should be restored, potentially by:

1. Adding return place initialization tracking to the borrow checker
2. Making it part of the type checking phase in a different way
3. Adding it as a separate validation pass