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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> > I would like to avoid adding additional special properties to AS0, or defining the flat concept.
> 
> How can we add a new specification w/o defining it?

By not defining it in terms of flat addressing. Just make it the undesirable address space

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