Add support for LLVM optimization remarks

I would like to map this information back to AST nodes, so that we can print remarks with spans, and so that remarks can be enabled on a per-function basis.  Unfortunately, doing this would require a lot more code restructuring — for example, we currently throw away the AST map and lots of other information before LLVM optimizations run.  So for the time being, we print the remarks with debug location strings from LLVM.  There's a warning if you use `-C remark` without `--debuginfo`.