Force `Rep` size to be at least 8 bytes when running under a sanitizer

Sanitizers are picky for container start addresses to be aligned at 8 byte aligned offsets, which causes issues under 32 bit compilations as we lay out the internal layout as {arena*, elements[...]}. This change forces the minimum header size to be at least 8 bytes, adding padding as needed.

PiperOrigin-RevId: 527596133

src/google/protobuf/repeated_field.h

@@ -423,17 +423,17 @@ class RepeatedField final
     return prev_size;
   }
 
-  // Pad the Rep after arena allow for power-of-two byte sizes when
-  // sizeof(Element) > sizeof(Arena*). eg for 16-byte objects.
-  static PROTOBUF_CONSTEXPR const size_t kRepHeaderSize =
-      sizeof(Arena*) < sizeof(Element) ? sizeof(Element) : sizeof(Arena*);
+  // Pad the rep to being max(Arena*, Element) with a minimum align
+  // of 8 as sanitizers are picky on the alignment of containers to
+  // start at 8 byte offsets even when compiling for 32 bit platforms.
   struct Rep {
-    Arena* arena;
-    Element* elements() {
-      return reinterpret_cast<Element*>(reinterpret_cast<char*>(this) +
-                                        kRepHeaderSize);
-    }
+    union {
+      alignas(8) Arena* arena;
+      Element unused;
+    };
+    Element* elements() { return reinterpret_cast<Element*>(this + 1); }
   };
+  static PROTOBUF_CONSTEXPR const size_t kRepHeaderSize = sizeof(Rep);
 
   // If total_size_ == 0 this points to an Arena otherwise it points to the
   // elements member of a Rep struct. Using this invariant allows the storage of