[clang] [Clang][CodeGen] Start migrating away from assuming the Default AS is 0 (PR #88182)

[Alex Voicu via cfe-commits]

AlexVlx wrote:

> > I'm not quite sure how to parse this comment, could you explain what you have in mind here? The problem is precisely that the FE assumes 0 is fine / picks it by default, which ends up into dangerzones when e.g. a target happened to use 0 to point to private (stack). I feel as if I'm missing the core of your comment though, so apologies in advance.
> 
> I'm just saying that I don't think it makes any sense to add a concept of a default AS to LLVM. The "default" AS is a frontend-level concept about how to interpret source-level types , not an LLVM-level concept. LLVM would only need a default AS if it were inventing a memory allocation/operation from whole cloth, which is generally not something LLVM should be doing except in local memory; the only legitimate counter-example I can think of would be something like materializing a constant into constant global memory, in which case LLVM needs to assign the new constant an AS.

Thinking about this a bit more, is it not the case that today, we do have a _de facto_ default AS in LLVM, if only by virtue of the fact that an unqualified ptr ends up as a ptr to AS 0; unqualified ptrs are used all over the place / the FE is pretty liberal in their employ? So, it's possible that a large part of this pain is that we say stuff like the below:
```cpp
/// void*, void** in address space 0
 union {
    llvm::PointerType *UnqualPtrTy;
```

https://github.com/llvm/llvm-project/pull/88182