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>