Improve lookup of operator<< for user types

Without this fix, trying to use this class with googletest

  struct Foo {};

  template <typename OutputStream>
  OutputStream& operator<<(OutputStream& os, const Foo&) {
    os << "TemplatedStreamableInFoo";
    return os;
  }

results in an ambiguity error between the class' operator<< and the
operator<< in gtest-printers.h removed in this CL.

This fix also enables implicit conversions to happen, so that e.g.
we will find the base class operator<< if a subclass has no
operator<< of its own.

PiperOrigin-RevId: 336261221