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

[Jay Foad via cfe-commits]

jayfoad wrote:

+1 to @efriedma-quic and @jdoerfert's comments. DataLayout should remain as generic as possible. Trying to encode a concept of "_the_ flat address space" in it seems way too specific to one optimization for one or two targets.

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