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

[Chris B via llvm-commits]

https://github.com/llvm-beanz commented:

I don't love the way you re-wrote clang/test/CodeGenHLSL/builtins/length.hlsl. I'm not a huge fan of the `update_cc_test_checks` workflow, and (as I commented elsewhere) I really don't like tests that depend on optimizations.

I'm okay with removing the `NO_HALF` side of the checks (I agree we had a lot of redundant test coverage there), and there's nothing unique in this change for half support.

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