[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:

> Would it be possible to update the SPIR-V DataLayout(s) as well please? Caveat, those targets use 4 to denote Flat/Generic, see e.g. [here](https://github.com/llvm/llvm-project/blob/47c3df2a7fcdfb33064d4d5e7d82dde1ea379023/llvm/lib/Target/SPIRV/SPIRVUtils.cpp#L176) and [here](https://github.com/llvm/llvm-project/blob/47c3df2a7fcdfb33064d4d5e7d82dde1ea379023/clang/lib/Basic/Targets/SPIR.h#L57).

Done.

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