1. Adding THD::convert_string(LEX_CSTRING *to,...) as a wrapper
   for convert_string(LEX_STRING *to,...), as LEX_CSTRING
   is now frequently used for conversion purpose.
   This reduced duplicate code in TEXT_STRING_sys,
   TEXT_STRING_literal, TEXT_STRING_filesystem grammar rules in *.yy

2. Adding yet another THD::convert_string() with an extra parameter
   "bool simple_copy_is_possible". This even more reduced
   repeatable code in the mentioned grammar rules in *.yy

3. Deriving Lex_ident_cli_st from Lex_string_with_metadata_st,
   as they have very similar functionality. Moving m_quote
   from Lex_ident_cli_st to Lex_string_with_metadata_st,
   as m_quote will be used later to optimize string literals anyway
   (e.g. avoid redundant copying on the tokenizer stage).
   Adjusting Lex_input_stream::get_text() accordingly.

4. Moving the reminders of the code in TEXT_STRING_sys, TEXT_STRING_literal,
   TEXT_STRING_filesystem grammar rules as new methods in THD:
   - make_text_string_sys()
   - make_text_string_connection()
   - make_text_string_filesystem()
   and changing *.yy to use these new methods.
   This reduced the amount of similar code in
   sql_yacc.yy and sql_yacc_ora.yy.

5. Removing duplicate code in Lex_input_stream::body_utf8_append_ident():
   by reusing THD::make_text_string_sys(). Thanks to #3 and #4.

6. Making THD members charset_is_system_charset,
   charset_is_collation_connection, charset_is_character_set_filesystem
   private, as they are not needed externally any more.