[clang] [HLSL] Implement the `ldexp` intrinsic (PR #138182)

[Farzon Lotfi via cfe-commits]

https://github.com/farzonl approved this pull request.

This looks fine.

 I would not like to carry forward native half testing of  intrinsics that are written purely in HLSL. I think the builtin ones aren't great either but I understand how they came to exist.

As for looking at notes. its easy to mess up the the vector size enforcements. and we have been using notes to confirm we did them right. I know for some notes we are going to have a bunch of repeated things we don't care about. But There is benefit to keeping them as there is no other way to confirm that the `no matching function for call to 'ldexp'` is happeing because of vector size out of bounds.

https://github.com/llvm/llvm-project/pull/138182