Fix memory leak of Rules, use bool phony_ member to identify phony edges

[jhasse]

Not sure if this is worth it and what the performance impact is.

[digit-google]

nit: Can we just store the address of this Rule in State::phony_rule_ and implement State::IsPhony(const Rule* rule) { return rule == phony_rule_; } instead? This would avoid adding 8 bytes to each Rule instance due to padding.

[jhasse]

you mean like it was before? That doesn't work anymore since then the Rule would be deleted twice in std::map<std::string, std::unique_ptr<const Rule>> rules_;, wouldn't it?

[digit-google]

Note necessarily with something like:

// state.h
struct State {
  ...
  const Rule* phony_rule() const { return phony_rule_; }
  
  Rule* phony_rule_ = nullptr;
  ...
};

// state.cc
State::State() {
  ...
  // Add the phony rule to it.
  phony_rule_ = new Rule("phony");
  bindings_.AddRule(phony_rule_);  // passes ownership to bindings_.
}

The caveat is that you need a State pointer whenever your want to implement rule->IsPhony(), but this is a minor modification to the rest of the code.

Frankly, this could go in a different PR. The memory leak fix is more important, so feel free to submit if you want.

[digit-google]

Apart from my nit, this LGTM.