[clang] [Clang][Doc] Modify generator for HLSL semantics documentation (PR #157841)

[Michał Górny via cfe-commits]

mgorny wrote:

Yeah, this seems to fix the issue I've hit.

For the record, I've finally gotten around to a simpler reproducer:

```
cmake ../llvm -G Ninja -DLLVM_ENABLE_PROJECTS='clang' -DCMAKE_BUILD_TYPE=MinSizeRel -DLLVM_CCACHE_BUILD=ON -DLLVM_ENABLE_SPHINX=ON
ninja gen-AttributeReference.rst
```

https://github.com/llvm/llvm-project/pull/157841