gccrs: Clean BiMap to use tl::optional for lookups

gccrs: Clean BiMap to use tl::optional for lookups

gcc/rust/Changelog:

	* expand/rust-expand-visitor.cc
	(ExpandVisitor::expand_inner_items): Adjust to use has_value ()
	(ExpandVisitor::expand_inner_stmts): Likewise
	* expand/rust-macro-builtins.cc (builtin_macro_from_string): Likewise
	(make_macro_path_str): Likewise
	* util/rust-hir-map.cc (Mappings::insert_macro_def): Likewise
	* util/rust-lang-item.cc (LangItem::Parse): Adjust to return tl::optional
	(LangItem::toString) Likewise
	* util/rust-token-converter.cc (handle_suffix): Adjust to use value.or ()
	(from_literal) Likewise
	* util/bi-map.h (BiMap::lookup): Adjust to use tl::optional for
	lookups

Signed-off-by: Sourabh Jaiswal <sourabhrj31@gmail.com>