Don't downcase non-ascii identifier chars in multi-byte encodings.

Long-standing code has called tolower() on identifier character bytes
with the high bit set. This is clearly an error and produces junk output
when the encoding is multi-byte. This patch therefore restricts this
activity to cases where there is a character with the high bit set AND
the encoding is single-byte.

There have been numerous gripes about this, most recently from Martin
Schäfer.

Backpatch to all live releases.

src/backend/parser/scansup.c

@@ -130,8 +130,10 @@ downcase_truncate_identifier(const char *ident, int len, bool warn)
 {
 	char	   *result;
 	int			i;
+	bool        enc_is_single_byte;
 
 	result = palloc(len + 1);
+	enc_is_single_byte = pg_database_encoding_max_length() == 1;
 
 	/*
 	 * SQL99 specifies Unicode-aware case normalization, which we don't yet
@@ -139,8 +141,8 @@ downcase_truncate_identifier(const char *ident, int len, bool warn)
 	 * locale-aware translation.  However, there are some locales where this
 	 * is not right either (eg, Turkish may do strange things with 'i' and
 	 * 'I').  Our current compromise is to use tolower() for characters with
-	 * the high bit set, and use an ASCII-only downcasing for 7-bit
-	 * characters.
+	 * the high bit set, as long as they aren't part of a multi-byte character,
+	 * and use an ASCII-only downcasing for 7-bit characters.
 	 */
 	for (i = 0; i < len; i++)
 	{
@@ -148,7 +150,7 @@ downcase_truncate_identifier(const char *ident, int len, bool warn)
 
 		if (ch >= 'A' && ch <= 'Z')
 			ch += 'a' - 'A';
-		else if (IS_HIGHBIT_SET(ch) && isupper(ch))
+		else if (enc_is_single_byte && IS_HIGHBIT_SET(ch) && isupper(ch))
 			ch = tolower(ch);
 		result[i] = (char) ch;
 	}