Preparatory refactoring for:

MDEV-6218 Wrong result of CHAR_LENGTH(non-BMP-character) with 3-byte utf8
- Moving get_text() as a method to Lex_input_stream.
- Moving the unescaping part into a separate function,
  this piece of code will later go to /strings most likely.
- Removing Lex_input_string::yytoklen, as it's not needed any more.

sql/sql_class.h

@@ -2960,6 +2960,10 @@ public:
     return (bool) (variables.sql_mode & (MODE_STRICT_TRANS_TABLES |
                                          MODE_STRICT_ALL_TABLES));
   }
+  inline bool backslash_escapes() const
+  {
+    return !MY_TEST(variables.sql_mode & MODE_NO_BACKSLASH_ESCAPES);
+  }
   inline my_time_t query_start() { query_start_used=1; return start_time; }
   inline ulong query_start_sec_part()
   { query_start_sec_part_used=1; return start_time_sec_part; }