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

[Sarah Spall 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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spall wrote:

A lot of the builtins that support half and float have tests with native half and without native half.
See: https://github.com/llvm/llvm-project/blob/main/clang/test/CodeGenHLSL/builtins/cos.hlsl
Should we do that here? 

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