[HLSL] Update `RootSignatures-AST.hlsl` to have full sample testcase once parser is complete

This task just tracks to update the following testcase with adding the full sample root signature as defined [here](https://learn.microsoft.com/en-us/windows/win32/direct3d12/specifying-root-signatures-in-hlsl#descriptor-table).

Testcase previously defined [here](https://github.com/llvm/llvm-project/pull/123985/files#diff-05ae53e9a3df79d46dc1a249bacc24a184bf9067aea4d42f57275f2aeef7a37b). 