[Instrumentation] Set hidden visibility for the bias variable

[phosek]

Summary

We have to avoid using a GOT relocation to access the bias variable,
setting the hidden visibility achieves that.

[mcgrathr]

There should be test coverage that verifies the visibility gets set, but lgtm with that.

[phosek]

Test updated

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

if (isRuntimeCounterRelocationEnabled()) {
     Type *Int64PtrTy = Type::getInt64PtrTy(M->getContext());
     Function *Fn = Inc->getParent()->getParent();
     Instruction &I = Fn->getEntryBlock().front();
     LoadInst *LI = dyn_cast<LoadInst>(&I);
     if (!LI) {
       IRBuilder<> Builder(&I);
       Type *Int64Ty = Type::getInt64Ty(M->getContext());
       GlobalVariable *Bias = M->getGlobalVariable(getInstrProfCounterBiasVarName());
-      if (!Bias)
+      if (!Bias) {
         Bias = new GlobalVariable(*M, Int64Ty, false, GlobalValue::LinkOnceODRLinkage,
                                   Constant::getNullValue(Int64Ty),
                                   getInstrProfCounterBiasVarName());
+        Bias->setVisibility(GlobalVariable::HiddenVisibility);
+      }
       LI = Builder.CreateLoad(Int64Ty, Bias);
     }
     auto *Add = Builder.CreateAdd(Builder.CreatePtrToInt(Addr, Int64Ty), LI);
     Addr = Builder.CreateIntToPtr(Add, Int64PtrTy);
   }
 
   if (Options.Atomic || AtomicCounterUpdateAll) {
     Builder.CreateAtomicRMW(AtomicRMWInst::Add, Addr, Inc->getStep(),