Update ref-op pruning pass to use remarks API for stats tracking

Following on from https://github.com/numba/llvmlite/pull/837, which adds support for obtaining remarks from LLVM passes. It should now be possible to update the `RefPrunePass`, which does Numba specific ref-op pruning, to emit remarks and for them to be consumed by this API. This would effectively replace this:
https://github.com/numba/llvmlite/blob/dfca79fe77d00264dd80a4955f206804c77719a0/ffi/custom_passes.cpp#L163-L166
and this:
https://github.com/numba/llvmlite/blob/dfca79fe77d00264dd80a4955f206804c77719a0/ffi/custom_passes.cpp#L1178-L1210
and also allowing tracking of the source line origin of the pruned ref-ops (which is interesting in itself).

Start with ffi/custom_passes.cpp at the linked RefPrunePass and statistics-handling ranges, then read the remarks support introduced by pull request #837. Determine how the pass should emit remarks and how the existing stats API consumes them, including source-line origin tracking. Done means the old tracking paths are replaced by remarks-based tracking.

Written by the indexing model from the issue text.