Also update psqlscan.l with the UESCAPE error rule changes.

Even though this patch had no user-visible difference, better keep the code
in psqlscan.l sync with the backend lexer. And of course it's nice to shrink
the psql binary, too. Ecpg's version of the lexer doesn't have the error
rule, it doesn't try to avoid backing up, so it doesn't need to be modified.

As reminded by Tom Lane

src/bin/psql/psqlscan.l

@@ -166,7 +166,8 @@ static void escape_variable(bool as_ident);
  *  <xe> extended quoted strings (support backslash escape sequences)
  *  <xdolq> $foo$ quoted strings
  *  <xui> quoted identifier with Unicode escapes
- *  <xus> quoted string with Unicode escapes
+ *  <xuiend> end of a quoted identifier with Unicode escapes, UESCAPE can follow *  <xus> quoted string with Unicode escapes
+ *  <xusend> end of a quoted string with Unicode escapes, UESCAPE can follow
  *
  * Note: we intentionally don't mimic the backend's <xeu> state; we have
  * no need to distinguish it from <xe> state, and no good way to get out
@@ -182,7 +183,9 @@ static void escape_variable(bool as_ident);
 %x xq
 %x xdolq
 %x xui
+%x xuiend
 %x xus
+%x xusend
 /* Additional exclusive states for psql only: lex backslash commands */
 %x xslashcmd
 %x xslashargstart
@@ -307,17 +310,17 @@ xdinside		[^"]+
 /* Unicode escapes */
 uescape			[uU][eE][sS][cC][aA][pP][eE]{whitespace}*{quote}[^']{quote}
 /* error rule to avoid backup */
-uescapefail		("-"|[uU][eE][sS][cC][aA][pP][eE]{whitespace}*"-"|[uU][eE][sS][cC][aA][pP][eE]{whitespace}*{quote}[^']|[uU][eE][sS][cC][aA][pP][eE]{whitespace}*{quote}|[uU][eE][sS][cC][aA][pP][eE]{whitespace}*|[uU][eE][sS][cC][aA][pP]|[uU][eE][sS][cC][aA]|[uU][eE][sS][cC]|[uU][eE][sS]|[uU][eE]|[uU])
+uescapefail		[uU][eE][sS][cC][aA][pP][eE]{whitespace}*"-"|[uU][eE][sS][cC][aA][pP][eE]{whitespace}*{quote}[^']|[uU][eE][sS][cC][aA][pP][eE]{whitespace}*{quote}|[uU][eE][sS][cC][aA][pP][eE]{whitespace}*|[uU][eE][sS][cC][aA][pP]|[uU][eE][sS][cC][aA]|[uU][eE][sS][cC]|[uU][eE][sS]|[uU][eE]|[uU]
 
 /* Quoted identifier with Unicode escapes */
 xuistart		[uU]&{dquote}
-xuistop1		{dquote}{whitespace}*{uescapefail}?
-xuistop2		{dquote}{whitespace}*{uescape}
 
 /* Quoted string with Unicode escapes */
 xusstart		[uU]&{quote}
-xusstop1		{quote}{whitespace}*{uescapefail}?
-xusstop2		{quote}{whitespace}*{uescape}
+
+/* Optional UESCAPE after a quoted string or identifier with Unicode escapes. */
+xustop1		{uescapefail}?
+xustop2		{uescape}
 
 /* error rule to avoid backup */
 xufailed		[uU]&
@@ -520,12 +523,22 @@ other			.
 					BEGIN(INITIAL);
 					ECHO;
 				}
-<xus>{xusstop1} {
+<xus>{quotestop} |
+<xus>{quotefail} {
 					yyless(1);
+					BEGIN(xusend);
+					ECHO;
+				}
+<xusend>{whitespace} {
+					ECHO;
+				}
+<xusend>{other} |
+<xusend>{xustop1} {
+					yyless(0);
 					BEGIN(INITIAL);
 					ECHO;
 				}
-<xus>{xusstop2} {
+<xusend>{xustop2} {
 					BEGIN(INITIAL);
 					ECHO;
 				}
@@ -612,12 +625,21 @@ other			.
 					BEGIN(INITIAL);
 					ECHO;
 				}
-<xui>{xuistop1}	{
+<xui>{dquote} {
 					yyless(1);
+					BEGIN(xuiend);
+					ECHO;
+				}
+<xuiend>{whitespace} {
+					ECHO;
+				}
+<xuiend>{other} |
+<xuiend>{xustop1} {
+					yyless(0);
 					BEGIN(INITIAL);
 					ECHO;
 				}
-<xui>{xuistop2}	{
+<xuiend>{xustop2}	{
 					BEGIN(INITIAL);
 					ECHO;
 				}