Use the assume attribute for gcc

ConflictSet.cpp

@@ -47,9 +47,7 @@ limitations under the License.
 #if __has_builtin(__builtin_assume)
 #define assume(e) __builtin_assume(e)
 #else
-#define assume(e)                                                              \
-  if (!(e))                                                                    \
-  __builtin_unreachable()
+#define assume(e) __attribute__((assume(e)))
 #endif
 #else
 #define assume assert