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

[Eli Friedman via cfe-commits]

efriedma-quic 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.

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