[llvm] [DebugInfo] Lower DW_OP_LLVM_implicit_pointer to DWARF (PR #186763)

[Jeremy Morse via llvm-commits]

jmorse wrote:

There's an awkward test gap here, in that llvm-dwarfdump on an unlinked binary won't print the relocations, all the expressions for me print as:

```
DW_AT_location        (DW_OP_implicit_pointer 0x0 +0)
```

thus we can't validate they point at the correct DIEs. Ideally the output would be linked, but that makes this more of an end-to-end test. I'm not sure if there's precedent for a solution here. 

I also note that because "Location list support (Loc::Multi) is not yet handled.", implicit-pointer expression components will be discarded if a location list is produced, leading to an incorrect variable location. Is there a PR for implementing these, or at least discarding the whole variable location if a location list is produced?

https://github.com/llvm/llvm-project/pull/186763