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

[Alexander Richardson via llvm-commits]

https://github.com/arichardson approved this pull request.

Would be good to fold clang/test/CodeGenCXX/vtable-assume-load-nonzero-default-address-space.cpp into one of the files it was copied from, otherwise LGTM.

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