[clang] [HLSL][RootSignature] Introduce `HLSLFrontendAction` to implement `rootsig-define` (PR #154639)

[Chris B via cfe-commits]

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@@ -9436,6 +9436,18 @@ def dxc_rootsig_ver :
   Alias<fdx_rootsignature_version>,
   Group<dxc_Group>,
   Visibility<[DXCOption]>;
+def fdx_rootsignature_define :
+  Joined<["-"], "fdx-rootsignature-define=">,
+  Group<dxc_Group>,
+  Visibility<[ClangOption, CC1Option]>,
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llvm-beanz wrote:

Since we have no tests that actually test that, I'm going to guess that's a copypasta mistake, not an intentional design decision. That said I'm okay leaving this as-is.

https://github.com/llvm/llvm-project/pull/154639