Add ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS helper.

PiperOrigin-RevId: 516245113
Change-Id: Idaef538c9392decbfdeb3ed3b3c109c795f92f4d

absl/base/attributes.h

@@ -779,4 +779,18 @@
 #define ABSL_ATTRIBUTE_TRIVIAL_ABI
 #define ABSL_ATTRIBUTE_TRIVIAL_ABI
 #endif
 #endif
 
 
+// ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS
+//
+// Indicates a data member can be optimized to occupy no space (if it is empty)
+// and/or its tail padding can be used for other members.
+//
+// For code that is assured to only build with C++20 or later, prefer using
+// the standard attribute `[[no_unique_address]]` directly instead of this
+// macro.
+#if ABSL_HAVE_CPP_ATTRIBUTE(no_unique_address)
+#define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS [[no_unique_address]]
+#else
+#define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS
+#endif
+
 #endif  // ABSL_BASE_ATTRIBUTES_H_
 #endif  // ABSL_BASE_ATTRIBUTES_H_