Fix pg_dump's failure to dump REPLICA IDENTITY for constraint indexes.

pg_dump knew about printing ALTER TABLE ... REPLICA IDENTITY USING INDEX
for indexes declared as indexes, but it failed to print that for indexes
declared as unique or primary-key constraints.  Per report from Achilleas
Mantzios.

This has been broken since the feature was introduced, AFAICS.
Back-patch to 9.4.

Discussion: https://postgr.es/m/1e6cc5ad-b84a-7c07-8c08-a4d0c3cdc938@matrix.gatewaynet.com

src/bin/pg_dump/pg_dump.c

@@ -15633,6 +15633,12 @@ dumpIndex(Archive *fout, IndxInfo *indxinfo)
 		/* Plain secondary index */
 		appendPQExpBuffer(q, "%s;\n", indxinfo->indexdef);
+		/*
+		 * Append ALTER TABLE commands as needed to set properties that we
+		 * only have ALTER TABLE syntax for.  Keep this in sync with the
+		 * similar code in dumpConstraint!
+		 */
+
 		/* If the index is clustered, we need to record that. */
 		if (indxinfo->indisclustered)
 		{
@@ -15833,6 +15839,12 @@ dumpConstraint(Archive *fout, ConstraintInfo *coninfo)
 			appendPQExpBufferStr(q, ";\n");
 		}
+		/*
+		 * Append ALTER TABLE commands as needed to set properties that we
+		 * only have ALTER TABLE syntax for.  Keep this in sync with the
+		 * similar code in dumpIndex!
+		 */
+
 		/* If the index is clustered, we need to record that. */
 		if (indxinfo->indisclustered)
 		{
@@ -15843,6 +15855,16 @@ dumpConstraint(Archive *fout, ConstraintInfo *coninfo)
 							  fmtId(indxinfo->dobj.name));
 		}
+		/* If the index defines identity, we need to record that. */
+		if (indxinfo->indisreplident)
+		{
+			appendPQExpBuffer(q, "\nALTER TABLE ONLY %s REPLICA IDENTITY USING",
+							  fmtQualifiedDumpable(tbinfo));
+			/* index name is not qualified in this syntax */
+			appendPQExpBuffer(q, " INDEX %s;\n",
+							  fmtId(indxinfo->dobj.name));
+		}
+
 		appendPQExpBuffer(delq, "ALTER TABLE ONLY %s ",
 						  fmtQualifiedDumpable(tbinfo));
 		appendPQExpBuffer(delq, "DROP CONSTRAINT %s;\n",