[clang] [lld] [llvm] [mlir] [IR] Introduce `T<address space>` to `DataLayout` to represent flat address space if a target supports it (PR #108786)

[Artem Belevich via cfe-commits]

Artem-B wrote:

>  I'm still concerned about the (no-)aliasing guarantees. It's useful to have two non-flat address spaces that can alias,

Another example for NVIDIA GPUs would be `.param` space. According to the [PTX spec](https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#generic-addressing) "The [Kernel Function Parameters](https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#kernel-function-parameters) (.param) window is contained within the .global window."  We could represent param as a subset of global AS, but it does look like we may need to be able to express aliasing or non-aliasing AS sets explicitly, too.

https://github.com/llvm/llvm-project/pull/108786