Update UCS_to_GB18030.pl with info about origin of the reference file.

src/backend/utils/mb/Unicode/UCS_to_GB18030.pl

@@ -5,7 +5,8 @@
 # src/backend/utils/mb/Unicode/UCS_to_GB18030.pl
 #
 # Generate UTF-8 <--> GB18030 code conversion tables from
-# "gb-18030-2000.xml"
+# "gb-18030-2000.xml", obtained from
+# http://source.icu-project.org/repos/icu/data/trunk/charset/data/xml/
 #
 # The lines we care about in the source file look like
 #    <a u="009A" b="81 30 83 36"/>

src/backend/utils/mb/Unicode/gb18030_to_utf8.map

+/* src/backend/utils/mb/Unicode/gb18030_to_utf8.map */
+
 static const pg_local_to_utf LUmapGB18030[ 30733 ] = {
   {0x8140, 0xe4b882},
   {0x8141, 0xe4b884},

src/backend/utils/mb/Unicode/utf8_to_gb18030.map

+/* src/backend/utils/mb/Unicode/utf8_to_gb18030.map */
+
 static const pg_utf_to_local ULmapGB18030[ 30733 ] = {
   {0xc280, 0x81308130},
   {0xc281, 0x81308131},