[Clang][OpenMP] Fix shared memory allocation on AMDGPU

[ZwFink]

Full details and reproducer at this GitHub issue: https://github.com/llvm/llvm-project/issues/60345

[tianshilei1992]

I think this looks reasonable to me. @jdoerfert WDYT? I'm not sure if you need to fix some clang tests. Let's see if Buildbot is happy.

[nlopes]

Please use poison values as place holders instead of undef values as we're trying to get rid of Undef. Thank you!

[ZwFink]

I wanted to follow up on this. Is anything additional needed from me?

[arsenm]

In D144590#4220648, @ZwFink wrote:

I wanted to follow up on this. Is anything additional needed from me?

This needs a test (I would hope there is already one that needs updating), and also should switch to using PoisonValue instead of UndefValue

clang/lib/CodeGen/CGOpenMPRuntimeGPU.cpp

 //===---- CGOpenMPRuntimeGPU.cpp - Interface to OpenMP GPU Runtimes ----===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
case OMPAllocateDeclAttr::OMPPTeamMemAlloc:
       break;
     case OMPAllocateDeclAttr::OMPLargeCapMemAlloc:
     case OMPAllocateDeclAttr::OMPCGroupMemAlloc:
       break;
     }
     llvm::Type *VarTy = CGF.ConvertTypeForMem(VD->getType());
     auto *GV = new llvm::GlobalVariable(
         CGM.getModule(), VarTy, /*isConstant=*/false,
-        llvm::GlobalValue::InternalLinkage, llvm::Constant::getNullValue(VarTy),
+        llvm::GlobalValue::InternalLinkage, llvm::UndefValue::get(VarTy),
         VD->getName(),
         /*InsertBefore=*/nullptr, llvm::GlobalValue::NotThreadLocal,
         CGM.getContext().getTargetAddressSpace(AS));
     CharUnits Align = CGM.getContext().getDeclAlign(VD);
     GV->setAlignment(Align.getAsAlign());
     return Address(
         CGF.Builder.CreatePointerBitCastOrAddrSpaceCast(
             GV, VarTy->getPointerTo(CGM.getContext().getTargetAddressSpace(