[OpenMP][AMDGPU] Refactor setting uniform work group size attribute

[domada]

Summary

Work group size attribute was set in Clang specific class. That's why we cannot reuse this code in Flang.

If we move setting of this attribute to OpenMPIRBuilder, then we can reuse this code in Flang and Clang. Function createOffloadEntry from OpenMPIRBuilder is already used by Clang (via OpenMPIRBuilder::createOffloadEntriesAndInfoMetadata function).

clang/lib/CodeGen/TargetInfo.cpp

if (!F)
     return;
 
   const FunctionDecl *FD = dyn_cast_or_null<FunctionDecl>(D);
   if (FD)
     setFunctionDeclAttributes(FD, F, M);
 
   const bool IsHIPKernel =
       M.getLangOpts().HIP && FD && FD->hasAttr<CUDAGlobalAttr>();
-  const bool IsOpenMPkernel =
-      M.getLangOpts().OpenMPIsDevice &&
-      (F->getCallingConv() == llvm::CallingConv::AMDGPU_KERNEL);
 
   // TODO: This should be moved to language specific attributes instead.
-  if (IsHIPKernel || IsOpenMPkernel)
+  if (IsHIPKernel)
     F->addFnAttr("uniform-work-group-size", "true");
 
   if (M.getContext().getTargetInfo().allowAMDGPUUnsafeFPAtomics())
     F->addFnAttr("amdgpu-unsafe-fp-atomics", "true");
 
   if (!getABIInfo().getCodeGenOpts().EmitIEEENaNCompliantInsts)
     F->addFnAttr("amdgpu-ieee", "false");
 }

llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp

void OpenMPIRBuilder::createOffloadEntry(Constant *ID, Constant *Addr,
   Metadata *MDVals[] = {
       ConstantAsMetadata::get(Fn), MDString::get(Ctx, "kernel"),
       ConstantAsMetadata::get(ConstantInt::get(Type::getInt32Ty(Ctx), 1))};
   // Append metadata to nvvm.annotations.
   MD->addOperand(MDNode::get(Ctx, MDVals));
 
   // Add a function attribute for the kernel.
   Fn->addFnAttr(Attribute::get(Ctx, "kernel"));
+  if (Triple(M.getTargetTriple()).isAMDGCN())
+    Fn->addFnAttr("uniform-work-group-size", "true");
 }
 
 // We only generate metadata for function that contain target regions.
 void OpenMPIRBuilder::createOffloadEntriesAndInfoMetadata(
     EmitMetadataErrorReportFunctionTy &ErrorFn) {
 
   // If there are no entries, we don't need to do anything.
   if (OffloadInfoManager.empty())