- Using Lex_ident_sys to scan identifiers, like the SQL parser does.

  This fixes handling of double-quote-delimited and backtick-delimited identifiers,
  as well as handling of non-ASCII identifiers.

  Unescaping and converting from the client character set to the system
  character set is now done using Lex_ident_cli_st and Lex_ident_sys,
  like it's done in the SQL tokenizer/parser.
  Adding helper methods to_ident_cli() and to_ident_sys()
  in Optimizer_hint_parser::Token.

- Fixing the hint parser to report a syntax error when an empty identifiers:
    SELECT /*+ BKA(``) */ * FROM t1;

- Moving a part of the code from opt_hints_parser.h to opt_hints_parser.cc

  Moving these method definitions:
  - Optimizer_hint_tokenizer::find_keyword()
  - Optimizer_hint_tokenizer::get_token()

  to avoid huge pieces of the code in the header file.

- A Lex_ident_cli_st cleanup
  Fixing a few Lex_ident_cli_st methods to return Lex_ident_cli_st &
  instead of void, to use them easier in the caller code.

- Fixing the hint parser to display the correct line number

  Adding a new data type Lex_comment_st
  (a combination of LEX_CSTRING and a line number)
  Using it in sql_yacc.yy

- Getting rid of redundant dependencies on sql_hints_parser.h

  Moving void LEX::resolve_optimizer_hints() from sql_lex.h to sql_lex.cc

  Adding a class Optimizer_hint_parser_output, deriving from
  Optimizer_hint_parser::Hint_list. Fixing the hint parser to
  return a pointer to an allocated instance of Optimizer_hint_parser_output
  rather than an instance of Optimizer_hint_parser::Hint_list.
  This allows to use a forward declaration of Optimizer_hint_parser_output
  in sql_lex.h and thus avoid dependencies on sql_hints_parser.h.