Introduce detected_formatter to IR

[copybara-service]

Introduce detected_formatter to IR

Detects the two main ways in google3 to format a type in C++:

• AbslStringify
• operator<<(std::ostream&)

Should work for common use cases, though unsupported edge cases (for now) include:

• Correctly handling constrained template parameters e.g.,

  template <typename T>
  requires (absl::HasOstreamOperator<T>::value)
  std::ostream& operator<<(std::ostream&, const Foo<T>&);

• Templated std::basic_ostream e.g.,

  template <typename CharType, typename Traits>
  std::basic_ostream<CharType, Traits>&
  operator<<(std::basic_ostream<CharType, Traits>&, const Foo&);

• Correctly handling delete-d functions

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