[llvm] [NFC] Refactoring MCDXBC to support out of order storage of root parameters (PR #137284)

[via llvm-commits]

================
@@ -75,31 +75,32 @@ static bool parseRootConstants(LLVMContext *Ctx, mcdxbc::RootSignatureDesc &RSD,
   if (RootConstantNode->getNumOperands() != 5)
     return reportError(Ctx, "Invalid format for RootConstants Element");
 
-  mcdxbc::RootParameter NewParameter;
-  NewParameter.Header.ParameterType =
+  dxbc::RootParameterHeader Header;
+  Header.ParameterType =
       llvm::to_underlying(dxbc::RootParameterType::Constants32Bit);
 
   if (std::optional<uint32_t> Val = extractMdIntValue(RootConstantNode, 1))
-    NewParameter.Header.ShaderVisibility = *Val;
+    Header.ShaderVisibility = *Val;
   else
     return reportError(Ctx, "Invalid value for ShaderVisibility");
----------------
joaosaffran wrote:

There is no need to initialized `Header.ParameterOffset`, since those will be calculated when writing, in `RootSignatureDesc::write`

https://github.com/llvm/llvm-project/pull/137284