Fix False positive "Function declared to return Never but is missing an explicit return"

[mwestbroek]

Summary

When a function annotation is -> Never and there is no explicit return in the body, don't warn about lacking an explicit return.

In practice, we don't proceed to check_type() in check_implicit_return_against_annotation() since got and want are indeed mismatched but we don't want to emit an error.

Fixes #4176.

Test Plan

Added test_no_explicit_return_if_return_type_is_never to lib/test/returns.rs which fails before the change in lib/alt/solve.rs, and passes afterwards.

I've run test.py and committed the resulting changes - I am not sure how this should work so any feedback would be appreciated!

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