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/CGBlocks.cpp

@@ -915,7 +915,7 @@ llvm::Type *CodeGenModule::getBlockDescriptorType() {
   // };
   BlockDescriptorType =
     llvm::StructType::create("struct.__block_descriptor",
-                             UnsignedLongTy, UnsignedLongTy, NULL);
+                             UnsignedLongTy, UnsignedLongTy, nullptr);
 
   // Now form a pointer to that.
   BlockDescriptorType = llvm::PointerType::getUnqual(BlockDescriptorType);
@@ -938,7 +938,7 @@ llvm::Type *CodeGenModule::getGenericBlockLiteralType() {
   GenericBlockLiteralType =
     llvm::StructType::create("struct.__block_literal_generic",
                              VoidPtrTy, IntTy, IntTy, VoidPtrTy,
-                             BlockDescPtrTy, NULL);
+                             BlockDescPtrTy, nullptr);
 
   return GenericBlockLiteralType;
 }