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

[Finn Plummer via cfe-commits]

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@@ -34,3 +34,7 @@ void bad_root_signature_5() {}
 // expected-error at +1 {{expected ')' to denote end of parameters, or, another valid parameter of RootConstants}}
 [RootSignature(MultiLineRootSignature)]
 void bad_root_signature_6() {}
+
+// expected-error at +1 {{expected end of stream to denote end of parameters, or, another valid parameter of RootSignature}}
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inbelic wrote:

Yes. I think we can simplify the diagnostic here quite a bit.

A similar concern was also noted here: https://github.com/llvm/llvm-project/pull/145827#discussion_r2169406679

I will create a follow-up issue tomorrow to track this work and do so in a follow-up pr, but will leave this pr to just focus on the bug fix as it has a dependency here: https://github.com/llvm/llvm-project/pull/147115#discussion_r21929896965.

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