[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:

> But that will still require to define, what is undesirable address space right?

The LangRef doesn't need to know why it's undesirable. It's like the n field

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