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

[Farzon Lotfi via cfe-commits]

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+// RUN: %clang_cc1 -finclude-default-header -x hlsl -triple dxil-pc-shadermodel6.3-library %s -fnative-half-type -emit-llvm -disable-llvm-passes -o - | FileCheck %s
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farzonl wrote:

I was hoping we would only do those for intrinsics that require clang builtins since we define most of them as `void(...)`.   for anything thats pure HLSL like this one we have extensive testing of native half.

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