[clang] [HLSL][RootSignature] Implement diagnostic for missed comma (PR #147350)

[Justin Bogner via cfe-commits]

https://github.com/bogner commented:

What does this do when there are commas at the ends of lists of elements?

Interestingly, DXC seems inconsistent on its behaviour for those:
```hlsl
// Unexpected token ')'
[RootSignature("CBV(b0), CBV(b1,)")]
// valid
[RootSignature("CBV(b0), CBV(b1),")]
```

I don't know that we need to match this exactly - we should probably be consistent about it. In any case, please do add some tests that make sure we do something sensible.

https://github.com/llvm/llvm-project/pull/147350