De-duplicate moved variable errors.

By introducing a new map that tracks the errors reported and the
`Place`s that spawned those errors against the move out that the error
was referring to, we are able to silence duplicate errors by emitting
only the error which corresponds to the most specific `Place` (that which
other `Place`s which reported errors are prefixes of).

This generally is an improvement, however there is a case -
`liveness-move-in-while` - where the output regresses.

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