[clang] [llvm] [HLSL] Move length support out of the DirectX Backend (PR #121611)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

> Thanks for the description. I would find some justification useful even if it just repeats the reasoning we've established for all such changes.

@pow2clk added a justification.

https://github.com/llvm/llvm-project/pull/121611