Add support for REINDEX in event triggers

This commit adds support for REINDEX in event triggers, making this
command react for the events ddl_command_start and ddl_command_end.  The
indexes rebuilt are collected with the ReindexStmt emitted by the
caller, for the concurrent and non-concurrent paths.

Thanks to that, it is possible to know a full list of the indexes that a
single REINDEX command has worked on.

Author: Garrett Thornburg, Jian He
Reviewed-by: Jim Jones, Michael Paquier
Discussion: https://postgr.es/m/CAEEqfk5bm32G7sbhzHbES9WejD8O8DCEOaLkxoBP7HNWxjPpvg@mail.gmail.com

src/backend/tcop/utility.c

@@ -960,10 +960,6 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 							  (RecoveryInProgress() ? 0 : CHECKPOINT_FORCE));
 			break;
 
-		case T_ReindexStmt:
-			ExecReindex(pstate, (ReindexStmt *) parsetree, isTopLevel);
-			break;
-
 			/*
 			 * The following statements are supported by Event Triggers only
 			 * in some cases, so we "fast path" them in the other cases.
@@ -1574,6 +1570,13 @@ ProcessUtilitySlow(ParseState *pstate,
 				}
 				break;
 
+			case T_ReindexStmt:
+				ExecReindex(pstate, (ReindexStmt *) parsetree, isTopLevel);
+
+				/* EventTriggerCollectSimpleCommand is called directly */
+				commandCollected = true;
+				break;
+
 			case T_CreateExtensionStmt:
 				address = CreateExtension(pstate, (CreateExtensionStmt *) parsetree);
 				break;