Never return uninhabited values at all

Functions with uninhabited return values are already marked `noreturn`,
but we were still generating return instructions for this. When running
with `-C passes=lint`, LLVM prints:

    Unusual: Return statement in function with noreturn attribute

The LLVM manual makes a stronger statement about `noreturn` though:

> This produces undefined behavior at runtime if the function ever does
dynamically return.

We now emit an `abort` anywhere that would have tried to return an
uninhabited value.

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