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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

My impression is that the treatment of address-spaces varies so much across different languages and different targets that it doesn't make sense to encode the entire address-space layout the datalayout.  Narrowly encoding just the specific properties optimizations need means we don't need to rework optimizations and the datalayout for each new generation of GPU/accelerator.  And developers working on IR optimizations don't need to try to decipher the implications of different address-spaces.

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