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

[John McCall via cfe-commits]

rjmccall wrote:

> Why can't we just declare that the "generic" address-space must always be 0? The specific numbers we use for address-spaces are completely arbitrary anyway.

I don't think this works; there's nothing that LLVM could possibly do with a generic AS that would justify it being called out in the representation like this, because the existence of a generic AS is a deeply non-portable concept.  The only AS that I think would be worthwhile to call out like this would be the `alloca` AS, and in fact I tried to argue that as a reason not to introduce the `alloca` AS to `DataLayout` in the first place.  Unfortunately, I lost that argument, IIRC because some targets were adamant that they wanted to vary the `alloca` AS between functions.

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