[llvm] [ADT] Implement ArrayRef::operator< and other comparisons (PR #147277)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Looks good, but can you show a usecase?

I'm working on a patch that uses it [here](https://github.com/llvm/llvm-project/blob/2bb72924c245e328e08dde546e5377273bd731f4/llvm/utils/TableGen/Common/CodeGenRegisters.cpp#L921), changing `Members` from a pointer-to-SmallVector to an ArrayRef. I guess this patch can wait until that one is ready for review.

https://github.com/llvm/llvm-project/pull/147277