[clang] [HLSL] Support packoffset attribute in AST (PR #89836)

[Chris B via cfe-commits]

================
@@ -108,3 +108,18 @@ behavior between Clang and DXC. Some examples include:
   diagnostic notifying the user of the conversion rather than silently altering
   precision relative to the other overloads (as FXC does) or generating code
   that will fail validation (as DXC does).
+
+Mix packoffset and non-packoffset
+=================================
+
+DXC allows mixing packoffset and non-packoffset elements in the same cbuffer,
+accompanied by a warning.
+
+However, both Clang and FXC do not permit this and instead report an error.
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llvm-beanz wrote:

I worry about this. I thought we had discussed this and DXC did something sane and reasonable so we could do the same in Clang. Is there some detail I'm missing as to why we can't match DXC.

(cc @tex3d)

https://github.com/llvm/llvm-project/pull/89836