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

[John McCall via cfe-commits]

rjmccall 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 concept, not a middle-end / back-end 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 ever be doing; the only legitimate counter-example I can think of would be something like materializing a constant into constant global memory.

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