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

[Alexander Richardson via cfe-commits]

https://github.com/arichardson requested changes to this pull request.

The code change itself looks good to me and will be helpful for e.g. the CHERI fork.

However, I don't think copy-pasting tests is a good strategy it would be much cleaner to just add a new RUN line to the existing tests.

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