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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> > > querying a modules global AS from the target, rather than from the data layout (some DL's are incomplete, e.g. SPIRV's)
> 
> That is a bug in those DataLayouts
> 
> Do we spell out the requirement somewhere? I am only asking because, for example, [neither SPIR nor SPIRV have a complete DL string](https://github.com/llvm/llvm-project/blob/main/clang/lib/Basic/Targets/SPIR.h).

The section on datalayouts in LangRef explains that the specified datalayout is required to match the backend.

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