Fix length checking for Unicode identifiers containing escapes (U&"...").

We used the length of the input string, not the de-escaped string, as
the trigger for NAMEDATALEN truncation.  AFAICS this would only result
in sometimes printing a phony truncation warning; but it's just luck
that there was no worse problem, since we were violating the API spec
for truncate_identifier().  Per bug #9204 from Joshua Yanovski.

This has been wrong since the Unicode-identifier support was added,
so back-patch to all supported branches.

src/backend/parser/scan.l

@@ -697,13 +697,15 @@ other			.
 				}
 <xui>{xuistop1}	{
 					char	   *ident;
+					int			identlen;
 
 					BEGIN(INITIAL);
 					if (yyextra->literallen == 0)
 						yyerror("zero-length delimited identifier");
 					ident = litbuf_udeescape('\\', yyscanner);
-					if (yyextra->literallen >= NAMEDATALEN)
-						truncate_identifier(ident, yyextra->literallen, true);
+					identlen = strlen(ident);
+					if (identlen >= NAMEDATALEN)
+						truncate_identifier(ident, identlen, true);
 					yylval->str = ident;
 					/* throw back all but the quote */
 					yyless(1);
@@ -711,13 +713,15 @@ other			.
 				}
 <xui>{xuistop2}	{
 					char	   *ident;
+					int			identlen;
 
 					BEGIN(INITIAL);
 					if (yyextra->literallen == 0)
 						yyerror("zero-length delimited identifier");
 					ident = litbuf_udeescape(yytext[yyleng - 2], yyscanner);
-					if (yyextra->literallen >= NAMEDATALEN)
-						truncate_identifier(ident, yyextra->literallen, true);
+					identlen = strlen(ident);
+					if (identlen >= NAMEDATALEN)
+						truncate_identifier(ident, identlen, true);
 					yylval->str = ident;
 					return IDENT;
 				}