Use nullptr to silence -Wsentinel when self-hosting on Windows

Richard rejected my Sema change to interpret an integer literal zero in
a varargs context as a null pointer, so -Wsentinel sees an integer
literal zero and fires off a warning. Only CodeGen currently knows that
it promotes integer literal zeroes in this context to pointer size on
Windows.  I didn't want to teach -Wsentinel about that compatibility
hack. Therefore, I'm migrating to C++11 nullptr.

llvm-svn: 223079

clang/lib/CodeGen/CodeGenModule.cpp

@@ -624,7 +624,7 @@ void CodeGenModule::EmitCtorList(const CtorList &Fns, const char *GlobalName) {
 
   // Get the type of a ctor entry, { i32, void ()*, i8* }.
   llvm::StructType *CtorStructTy = llvm::StructType::get(
-      Int32Ty, llvm::PointerType::getUnqual(CtorFTy), VoidPtrTy, NULL);
+      Int32Ty, llvm::PointerType::getUnqual(CtorFTy), VoidPtrTy, nullptr);
 
   // Construct the constructor and destructor arrays.
   SmallVector<llvm::Constant*, 8> Ctors;