[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:

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

`n` field? The following?

```
n<size1>:<size2>:<size3>...
This specifies a set of native integer widths for the target CPU in bits. For example, it might contain n32 for 32-bit PowerPC, n32:64 for PowerPC 64, or n8:16:32:64 for X86-64. Elements of this set are considered to support most general arithmetic operations efficiently.
```

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