Consider using AssignmentTracking / dbg_assign

[nikic]

LLVM 16 introduced a new dbg_assign mechanism for representing debug info, that promises more accurate results, see:
https://llvm.org/docs/AssignmentTracking.html
https://discourse.llvm.org/t/rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir/62367

If I understand correctly, this can be enabled by scheduling an additional pass, without having to change the debug emission in the frontend: https://github.com/llvm/llvm-project/blob/7d32d72f10445657f598cbdf1bb5f3040373daf5/clang/lib/CodeGen/BackendUtil.cpp#L937-L947

We should consider whether we want to make use of this. Clang uses it for optimized, non-LTO builds by default.