optimization.h: Add missing <utility> header for C++

PiperOrigin-RevId: 646172195
Change-Id: I089f1d84f2d73b663f12e6818f96436e054e71ae

absl/base/optimization.h

@@ -18,12 +18,23 @@
 // -----------------------------------------------------------------------------
 // -----------------------------------------------------------------------------
 //
 //
 // This header file defines portable macros for performance optimization.
 // This header file defines portable macros for performance optimization.
+//
+// This header is included in both C++ code and legacy C code and thus must
+// remain compatible with both C and C++. C compatibility will be removed if
+// the legacy code is removed or converted to C++. Do not include this header in
+// new code that requires C compatibility or assume C compatibility will remain
+// indefinitely.
 
 
 #ifndef ABSL_BASE_OPTIMIZATION_H_
 #ifndef ABSL_BASE_OPTIMIZATION_H_
 #define ABSL_BASE_OPTIMIZATION_H_
 #define ABSL_BASE_OPTIMIZATION_H_
 
 
 #include <assert.h>
 #include <assert.h>
 
 
+#ifdef __cplusplus
+// Included for std::unreachable()
+#include <utility>
+#endif  // __cplusplus
+
 #include "absl/base/config.h"
 #include "absl/base/config.h"
 #include "absl/base/options.h"
 #include "absl/base/options.h"