DeadFunctionElimination: remove externally available witness tables at the end of the pipeline #29472
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... including all SIL functions with are transitively referenced from such witness tables.
After the last devirtualizer run, witness tables are not needed in the optimizer anymore.
We can delete witness tables with an available-externally linkage. IRGen does not emit such witness tables anyway.
This can save a little bit of compile time, because it reduces the amount of SIL at the end of the optimizer pipeline.
It also reduces the size of the SIL output after the optimizer, which makes debugging the SIL output easier.