gccrs: Improve lexer dump

gcc/rust/ChangeLog:

	* lex/rust-lex.cc (Lexer::Lexer): Add `dump_lex` boolean flag.
	(Lexer::skip_token): Dump tokens if flag is enabled.
	(Lexer::dump_and_skip): New function.
	* lex/rust-lex.h: Include optional.h and declare functions.
	* parse/rust-parse-impl.h (Parser::debug_dump_lex_output): Remove old
	unused function.
	* parse/rust-parse.h: Likewise.
	* rust-session-manager.cc (Session::compile_crate): Pass lexer dump
	option to lexer.
	(Session::dump_lex): New function.
	* util/rust-optional.h: Add missing constructor.

Signed-off-by: Raiki Tamura <tamaron1203@gmail.com>