[clang] [HLSL] Add the `Frs` `DXC` driver option (PR #157690)

[Finn Plummer via cfe-commits]

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@@ -0,0 +1,10 @@
+// RUN: %clang_dxc -T cs_6_0 /Fo %t.dxo /Frs %t.rs.dxo -### %s 2>&1 | FileCheck %s
+
+// Test to demonstrate that extracting the root signature to the specified
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inbelic wrote:

```suggestion
// Test to demonstrate extracting the root signature to the specified
```

https://github.com/llvm/llvm-project/pull/157690