[clang] [llvm] Add length HLSL function to DirectX Backend (PR #101256)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

> This PR adds the length intrinsic and an HLSL function that uses it. The SPIRV implementation is left for a future PR. Fixes #99134

You don't  want to mark this as fixed because it will close out this ticket and you still need to address the SPIRV backend changes.

https://github.com/llvm/llvm-project/pull/101256