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

[Justin Bogner via llvm-commits]

================
@@ -9,18 +9,81 @@
 #include "llvm/BinaryFormat/DXContainer.h"
 #include <cstdint>
 #include <limits>
+#include <variant>
 
 namespace llvm {
 
 class raw_ostream;
 namespace mcdxbc {
 
-struct RootParameter {
+struct RootParameterInfo {
   dxbc::RootParameterHeader Header;
-  union {
-    dxbc::RootConstants Constants;
-    dxbc::RTS0::v2::RootDescriptor Descriptor;
-  };
+  size_t Location;
+
+  RootParameterInfo() = default;
+
+  RootParameterInfo(dxbc::RootParameterHeader H, size_t L)
+      : Header(H), Location(L) {}
+};
+
+using RootDescriptor = std::variant<dxbc::RTS0::v1::RootDescriptor,
+                                    dxbc::RTS0::v2::RootDescriptor>;
+using ParametersView = std::variant<const dxbc::RootConstants *,
+                                    const dxbc::RTS0::v1::RootDescriptor *,
+                                    const dxbc::RTS0::v2::RootDescriptor *>;
+struct RootParametersContainer {
+  SmallVector<RootParameterInfo> ParametersInfo;
+
+  SmallVector<dxbc::RootConstants> Constants;
+  SmallVector<RootDescriptor> Descriptors;
----------------
bogner wrote:

We're not tied to the binary format itself here, so I think it would be quite a bit simpler to just have this vector contain the `v2` descriptors:
```c++
  SmallVector<dxbc::RTS0::v2::RootDescriptor> Descriptors;
```

This should end up simplifying `RootSignatureDesc::write`, as we have the version there, so we can just check the version for the later fields when we write it out:
```c++
      support::endian::write(BOS, Descriptor.ShaderRegister,
                             llvm::endianness::little);
      support::endian::write(BOS, Descriptor.RegisterSpace,
                             llvm::endianness::little);
      if (Version > 1)
        support::endian::write(BOS, Descriptor.Flags, llvm::endianness::little);
```

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