Avoid side-effects from `try_coerce` when suggesting borrowing LHS of cast
The name `try_coerce` is a bit misleading -- it has side-effects, so when it's used in diagnostics code, it sometimes causes spurious obligations to be registered which cause other errors to occur that really make no sense in context.
Addendum: let's just rename `try_coerce` to `coerce` -- the `try_` part doesn't really add much, imo.
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