[clang] [HLSL][Matrix] Add support for Matrix element and trunc Casts (PR #168915)

[Chris B via cfe-commits]

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+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.6-library -finclude-default-header -Wmatrix-conversion -verify %s
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llvm-beanz wrote:

```suggestion
// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.6-library -finclude-default-header -verify %s
```
This should be on by default right?

https://github.com/llvm/llvm-project/pull/168915