[clang] [llvm] [HLSL] Implement the `faceforward` intrinsic (PR #135878)

[Finn Plummer via llvm-commits]

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@@ -0,0 +1,94 @@
+// RUN: %clang_cc1 -finclude-default-header -x hlsl -triple \
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inbelic wrote:

```suggestion
// RUN: %clang_cc1 -finclude-default-header -triple \
```
`-x` just specifies the file extension, since the file is `.hlsl` it should naturally detect and use it as expected.

https://github.com/llvm/llvm-project/pull/135878