[llvm] [AMDGPU][Attributor] Make `AAAMDFlatWorkGroupSize` honor existing attribute (PR #114357)

[Shilei Tian via llvm-commits]

shiltian wrote:

@jtramm You are building vanilla LLVM but the device-libs are in [AMD's fork](https://github.com/ROCm/llvm-project/tree/amd-staging/amd/device-libs). You can simply use the LLVM you build to build device-libs. Here is the command I usually use to build device-libs:

```
cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DLLVM_ROOT=... -DClang_ROOT=... -B $BUILD_ROOT/device-libs/release -S llvm-project/amd/device-libs
```

https://github.com/llvm/llvm-project/pull/114357