Initial part of adding nullability annotations to C++ generated code

Add protobuf local macro similar to the very recently added absl version abseil/abseil-cpp@cd9dd42 The macro form allows Wnullability-completeness to check (vs absl's template form of annotation)

The macro will be empty unless you define PROTOBUF_TEMPORARY_NULL_ANNOTATIONS. We will switch that on once all annotations are in. The annotations will also be off in open source until we are ready.

PiperOrigin-RevId: 711805580

src/google/protobuf/compiler/cpp/generator.cc

@@ -67,6 +67,9 @@ absl::flat_hash_map<absl::string_view, std::string> CommonVars(
       {"hrule_thick", kThickSeparator},
       {"hrule_thin", kThinSeparator},
 
+      {"nullable", "PROTOBUF_NULLABLE"},
+      {"nonnull", "PROTOBUF_NONNULL"},
+
       // Warning: there is some clever naming/splitting here to avoid extract
       // script rewrites.  The names of these variables must not be things that
       // the extract script will rewrite.  That's why we use "CHK" (for example)

src/google/protobuf/port_def.inc

@@ -902,6 +902,14 @@ namespace internal {
 #define PROTOBUF_DEPRECATE_AND_INLINE() [[deprecated]]
 #endif
 
+#ifdef PROTOBUF_NONNULL
+#error PROTOBUF_NONNULL was previously defined
+#endif
+#ifndef PROTOBUF_NONNULL
+#define PROTOBUF_NONNULL
+#define PROTOBUF_NULLABLE
+#endif
+
 namespace google {
 namespace protobuf {
 namespace internal {

src/google/protobuf/port_undef.inc

@@ -63,6 +63,8 @@
 #undef PROTOBUF_TC_PARAM_DECL
 #undef PROTOBUF_DEBUG
 #undef PROTOBUF_NO_THREADLOCAL
+#undef PROTOBUF_NONNULL
+#undef PROTOBUF_NULLABLE
 
 #ifdef PROTOBUF_FUTURE_BREAKING_CHANGES
 #undef PROTOBUF_FUTURE_BREAKING_CHANGES