[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-shadermodel6.3-library -fnative-half-type -finclude-default-header -Wconversion -verify -o - %s -DERROR=1
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.3-library -fnative-half-type -finclude-default-header -Wno-conversion -ast-dump %s | FileCheck %s
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llvm-beanz wrote:

```suggestion
// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.3-library -fnative-half-type -finclude-default-header -verify -o - %s -DERROR=1
// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.3-library -fnative-half-type -finclude-default-header -ast-dump %s | FileCheck %s
```

The matrix warnings should be on by default so they shouldn't be impacted by or require `-Wconversion`.

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