Use nullptr instead of NULL for variadic sentinels

Windows defines NULL to 0, which when used as an argument to a variadic
function, is not a null pointer constant. As a result, Clang's
-Wsentinel fires on this code. Using '0' would be wrong on most 64-bit
platforms, but both MSVC and Clang make it work on Windows. Sidestep the
issue with nullptr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221940 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ShadowStackGC.cpp

@@ -144,7 +144,7 @@ namespace {
         LLVMContext &C = F.getContext();
         BasicBlock *CleanupBB = BasicBlock::Create(C, CleanupBBName, &F);
         Type *ExnTy = StructType::get(Type::getInt8PtrTy(C),
-                                      Type::getInt32Ty(C), NULL);
+                                      Type::getInt32Ty(C), nullptr);
         Constant *PersFn =
           F.getParent()->
           getOrInsertFunction("__gcc_personality_v0",