[OpenMP] Fix sema check for unified memory case NFC

[gtbercea]

Summary

This patch fixes a condition introduced in patch D60883.

[ABataev]

It does not look like NFC patch, the functionality has changed. Rename it, please, and a test case that reveals why we need this patch. Also, add a better description of the logic.

lib/Sema/SemaOpenMP.cpp

if (auto *VD = dyn_cast<VarDecl>(E->getDecl())) {
       DSAStackTy::DSAVarData DVar = Stack->getTopDSA(VD, /*FromParent=*/false);
       // Check if the variable has explicit DSA set and stop analysis if it so.
       if (DVar.RefExpr || !ImplicitDeclarations.insert(VD).second)
         return;
 
       // Skip internally declared static variables.
       llvm::Optional<OMPDeclareTargetDeclAttr::MapTypeTy> Res =
           OMPDeclareTargetDeclAttr::isDeclareTargetDeclaration(VD);
+      bool IsUsingUnifiedMemory =
+          Stack->hasRequiresDeclWithClause<OMPUnifiedSharedMemoryClause>();
       if (VD->hasGlobalStorage() && CS && !CS->capturesVariable(VD) &&
-          (Stack->hasRequiresDeclWithClause<OMPUnifiedSharedMemoryClause>() ||
-           !Res || *Res != OMPDeclareTargetDeclAttr::MT_Link))
+          ((!IsUsingUnifiedMemory &&
+            (!Res || *Res != OMPDeclareTargetDeclAttr::MT_Link)) ||
+           (IsUsingUnifiedMemory && !Res)))
         return;
 
       SourceLocation ELoc = E->getExprLoc();
       OpenMPDirectiveKind DKind = Stack->getCurrentDirective();
       // The default(none) clause requires that each variable that is referenced
       // in the construct, and does not have a predetermined data-sharing
       // attribute, must have its data-sharing attribute explicitly determined
       // by being listed in a data-sharing attribute clause.