[C++] Fix missing #ifdef for undefined identifiers (#9201)

* Fix #if for undefined identifiers

* port_undef.inc

src/google/protobuf/inlined_string_field.h

@@ -347,7 +347,7 @@ inline void InlinedStringField::Swap(
     InlinedStringField* from, const std::string* /*default_value*/,
     Arena* arena, bool donated, bool from_donated, uint32_t* donating_states,
     uint32_t* from_donating_states, uint32_t mask) {
-#if GOOGLE_PROTOBUF_INTERNAL_DONATE_STEAL_INLINE
+#ifdef GOOGLE_PROTOBUF_INTERNAL_DONATE_STEAL_INLINE
   // If one is donated and the other is not, undonate the donated one.
   if (donated && !from_donated) {
     MutableSlow(arena, donated, donating_states, mask);

src/google/protobuf/port_def.inc

@@ -617,7 +617,7 @@
 #ifdef PROTOBUF_PRAGMA_INIT_SEG
 #error PROTOBUF_PRAGMA_INIT_SEG was previously defined
 #endif
-#if _MSC_VER
+#ifdef _MSC_VER
 #define PROTOBUF_PRAGMA_INIT_SEG __pragma(init_seg(lib))
 #else
 #define PROTOBUF_PRAGMA_INIT_SEG
@@ -784,7 +784,7 @@
 #endif
 
 // Silence some MSVC warnings in all our code.
-#if _MSC_VER
+#ifdef _MSC_VER
 #pragma warning(push)
 // For non-trivial unions
 #pragma warning(disable : 4582)

src/google/protobuf/port_undef.inc

@@ -140,6 +140,6 @@
 #endif
 
 // Pop the warning(push) from port_def.inc
-#if _MSC_VER
+#ifdef _MSC_VER
 #pragma warning(pop)
 #endif