MDEV-19586: Rename recv_sys.empty() to recv_sys.clear()

In the collections of Standard Template Library,
empty() is a predicate and clear() empties a collection.
Let us rename recv_sys.empty() to recv_sys.clear() to avoid confusion.

storage/innobase/include/log0recv.h

@@ -304,8 +304,8 @@ struct recv_sys_t{
 			byte* body, byte* rec_end, lsn_t lsn,
 			lsn_t end_lsn);
 
-	/** Empty a fully processed set of stored redo log records. */
-	inline void empty();
+	/** Clear a fully processed set of stored redo log records. */
+	inline void clear();
 
 	/** Determine whether redo log recovery progress should be reported.
 	@param[in]	time	the current time

storage/innobase/log/log0recv.cc

@@ -814,8 +814,8 @@ void recv_sys_t::create()
 	last_stored_lsn = 0;
 }
 
-/** Empty a fully processed set of stored redo log records. */
-inline void recv_sys_t::empty()
+/** Clear a fully processed set of stored redo log records. */
+inline void recv_sys_t::clear()
 {
 	ut_ad(mutex_own(&mutex));
 	pages.clear();
@@ -2273,7 +2273,7 @@ void recv_apply_hashed_log_recs(bool last_batch)
 	recv_sys.apply_log_recs = false;
 	recv_sys.apply_batch_on = false;
 
-	recv_sys.empty();
+	recv_sys.clear();
 
 	mutex_exit(&recv_sys.mutex);
 }
@@ -3119,7 +3119,7 @@ recv_group_scan_log_recs(
 	mutex_enter(&recv_sys.mutex);
 	recv_sys.len = 0;
 	recv_sys.recovered_offset = 0;
-	recv_sys.empty();
+	recv_sys.clear();
 	srv_start_lsn = *contiguous_lsn;
 	recv_sys.parse_start_lsn = *contiguous_lsn;
 	recv_sys.scanned_lsn = *contiguous_lsn;