Refactor logic to check for ASCII-only characters in string

The same logic was present for collation commands, SASLprep and
pgcrypto, so this removes some code.

Author: Michael Paquier
Reviewed-by: Stephen Frost, Heikki Linnakangas
Discussion: https://postgr.es/m/X9womIn6rne6Gud2@paquier.xyz

src/common/string.c

@@ -92,6 +92,22 @@ pg_clean_ascii(char *str)
 }
 
 
+/*
+ * pg_is_ascii -- Check if string is made only of ASCII characters
+ */
+bool
+pg_is_ascii(const char *str)
+{
+	while (*str)
+	{
+		if (IS_HIGHBIT_SET(*str))
+			return false;
+		str++;
+	}
+	return true;
+}
+
+
 /*
  * pg_strip_crlf -- Remove any trailing newline and carriage return
  *