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

[Shilei Tian via cfe-commits]

shiltian wrote:

Thank all for the valuable input. I changed the wording in `LangRef.rst`. Hopefully it can make it more clear.

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